Missing one leg of a round trip flight

I'm confused about your post. Did you miss the first leg and find out upon check-in for the second leg that your your subsequent flights were cancelled? How did you get to the airport for your second flight?

One of my basic travel mantras is to BOOK DIRECTLY WITH THE PROVIDER WHENEVER POSSIBLE.

It will be in your airline's terms and conditions, sometimes called 'conditions of carriage'. You will have agreed to these when you bought the ticket and it was your responsibility in law to read them.

As an example, this is the relevant clause with British Airways:

3c10) If you fail to cancel a booking before the check-in deadline for your flight and do not show up for the flight, we may decide to cancel your return or onward reservations.

Some airlines book everything as singles where this wouldn't apply.

Wow, Sharon, you mean you didn't study and memorize Delta's 70 page Contract of Carriage? LOL

https://www.delta.com/content/dam/delta-www/pdfs/legal/contract_of_carriage_intl.pdf

I think Page 29 is what legally describes what happened to you:

3) Failure to Occupy Space If the passenger fails to occupy space which has been reserved for him on a flight of any carrier and such carrier fails to receive notice of the cancellation of such reservation prior to the departure of such flight, or if any carrier cancels the reservation of any passenger in accordance with paragraphs of this rule, such carrier will cancel all reservations held by such passenger on the flights of any carrier for continuing or return space, provided such carrier originally
reserved the space.

I'm sorry to hear this happened to you. I wouldn't expect rookie travelers to know this, but most seasoned travelers do. One reason airlines do this is to prevent people from trying to book cheaper fares on different routes where maybe the added connecting flight makes it cheaper. Say you want to fly from Philadelphia to Paris; there's a direct flight to Paris, but a flight from JFK that connects in Philly to the same plane that is $200 cheaper. Delta wants to prevent you from booking the cheaper flight then skipping the first leg. Also, of course, if you legitimately miss the first flight of a connection, you would probably miss the connecting flight, so it's more efficient for Delta to free up that seat for someone else.

I've actually done what they are trying to prevent to save the money, too. Last year, I needed a one-way flight from Helsinki to Amsterdam. It was about 90 Euros cheaper to add a return leg to Copenhagen instead of just booking the one-way. But because I was skipping out on the last flight not the first one, I got away with it. Technically, they may claim they can charge me after the fact for the higher fare, but in reality they don't.

I can imagine that Kayak wouldn't be on top of this, actually, and wouldn't expect them to notify you. This is one reason I prefer to book directly with the airlines unless I'm saving a ton of money by using some third party like Kayak: the middleman usually offers nothing but complications and higher fees to change a flight and can block legitimate communication between you and the airline.

It is pretty much common knowledge that it works that way, but like you say, you are not a seasoned traveler, and we all make rookie mistakes. Be aware, that French and Italian high speed trains work that way as well. They come with seat reservations, and if you miss your train, you can generally throw the ticket away, unless you paid full fare, where in you might have a 1 hour window to re-book on the next train, or get your money back less a service fee.

If something like this comes again, get on the phone immediately with the airlines. Agents sometimes have discretion to "break" the rules if you get on it right away. But coming in 24 hours after the flight has left gives them no options to do this.

Sorry this had to happen to you. I know it is water under the bridge (or over the dam) now. But now you (and any one else reading this post) knows better. C'est la vie.

You may want to sign up for a check-in alert.

If you had booked directly with the airlines, the same thing would have happened. It's just harder to know if there is anything to be done if you have more layers. Canceling the reservation is standard procedure if you miss the first leg of an itinerary. Not necessarily so for a subsequent legs, but absolutely for the first leg.

Airline tickets are route specific; they don't allow for any substitutions to the itinerary made by the ticket holder. That's why "change fees" apply for every change a ticket holder makes (a new ticket will likely have a totally different price). You can't skip out on a leg purposely or by accident and not void the entire ticket. The airline sees it as a breach of contract of its terms of carriage.

Booking directly with the airline instead of Kayak would have made no difference in your situation. Neither Kayak nor the airline had any obligation to make any additional notifications. The purchase of a ticket meant that you accepted the airline's contract terms, including all the small print (whether you were aware of it or not).

It stinks to learn this the hard way, but good luck in your future travels.

It won't help you any, but a quick note about Kayak: Many of the flights it lists on its search functions will be bumped to the operating airline and the customer is booking through the airline. Some other on-line agencies do book the flights for the customer. I have used Cheapoair with success (despite the name), especially for itineraries with more than one airline where they don't sell each other's tickets. But I do not trust such extras as seat assignments or flight notifications second hand. For those services I go straight to the airlines after making the purchase.

This why it would have been helpful to spend some time on this site prior to your trip. This subject is discussed from time to time.
https://community.ricksteves.com/travel-forum/spain/sfo-to-bcn-return-flight-thru-madrid-skip-bcn

No one is responsible for advising you that your ticket was cancelled. That has been a common policy for decades. Even with inexperience, logic might have asked the question about the return flight. Reading between the line - but did you just have a round trip ticket and you skipped the outbound flight? The airline is interested in having every seat filled. So if you are a no show on a RT ticket the assumption is that you are not going and, therefore, not coming back. So they are going to fill that seat if they can. Do you plan to skip the outbound leg in advance or was it caused by an accident?

Airlines have some discretion. Do a search on "airline flat tire rule".

Some airlines have an unwritten policy called the “Flat Tire Rule” in
which they'll let late passengers fly standby on the next flight
without having to pay to rebook. But you must show up within two hours
of your schedule departure.

Canceling the reservation is standard procedure if you miss the first leg of an itinerary. Not necessarily so for a subsequent legs, but absolutely for the first leg.

Actually if you miss ANY leg on most airlines all remaining flights are cancelled. The only exception is point to point airlines (SW in the US, EasyJet, Ryanair etc, in Europe for example. But your major carries, Delta, American, British Air, etc.... all practice that rule.)

I have the same curiosity as traylaparks as to how you got to the destination airport where you discovered the ticket had been canceled for your return flight?

Kayak's computer system had no way to know you missed your 1st flight. It just knew when the return flight was, so sent you a notice.

I always check my flight status starting a couple of weeks before to ensure everything is still good to go no matter who I book with.

I have the same curiosity as traylaparks as to how you got to the
destination airport where you discovered the ticket had been canceled
for your return flight?

My guess is that this ticket was purchased as a round trip (less expensive) but the intent was to only use the departing leg (meaning the OP was already at the departing city and didn't need to travel there from elsewhere). I can't think of any other way how a 24 hour + window would transpire between one leg of a flight and another in the case of a connecting flight. Most connecting flights don't have that large of a gap (it's a few hours max) unless it's something like a multi-day stopover in Iceland on Iceland Air or a 24 hour + stopover in Istanbul on Turkish Air.

I hope the OP will come back and clarify what exactly happened here. The posting makes it hard to decipher because the readers don't have all the information.

Edgar -- that is true but also very dependent on the gate agent. Years ago we booked a 12.30 am flight thinking it was 12.30pm. We checked in on-line, printed boarding passes, and still slept thought the am part. We showed up about 11 am looking for the 12.30 flight and it wasn't there. Since we had carry-on only the gate agent was our first human contact. He laughed and said we were not the first. It took stand-by on the next three flights but we final made it that afternoon. Now we are much more careful.

I zoned out and missed the date of a return flight by a day, but Air France just put us on the next flight back. My ticket was Delta, my husband's AF. The agent said we were lucky it was the return because an outbound would have been cancelled, but not the return.

@Frank, nice to know that even experienced travelers can misread a flight time. We were once driving a friend to his "1:21" return flight to Seattle -- when he realized the ticket really read 12:10, and he had missed the flight. (And this guy had been a flight attendant!)

Thank you for posting. I fly once in a while but would not have known this either. I have thankfully never missed a flight or had one cancelled on me yet. knocks on wood