lost (?stolen) *MINI* passport card (still have regular one)

https://travel.state.gov/content/travel/en/passports/have-passport/lost-stolen.html

You should only report your U.S. passport lost or stolen if the original, physical version of the passport book or passport card has been lost or stolen

As I read it you need to report the card missing/stolen and then begin the process to get a new passport

The advice was for you to be absolutely sure the card is really lost and not just hiding somewhere in a hard to get to spot in your luggage. IF you really have searched everywhere and you cannot find it then, yes it is time to report it as lost and go through the process and expense of getting a new passport (with or without the card) issued.

This is why I do not carry my passport card unless I am using it to enter and leave Canada.

The passport card has a different number than the passport and can only be used to re-enter the United States at land border-crossings and sea ports-of-entry from Canada, Mexico, the Caribbean, and Bermuda, for domestic travel and as ID for people like my 21-year-old daughter who want a proof of age document but refuse to learn how to drive. You can still travel on your regular passport if you lose the card. The reason the person who replied was so chill about the whole thing is that the card is pretty much useless other than as age-related ID and for the travel-related reasons mentioned above. I mean what entity gives out credit or loans or any other such thing based on a passport card?

The other, probably informal, use of the card is to prove that you have Global Entry. Conveniently stamped on the card. We have hit several port-of-entry that do not have Global Entry machines but have a Global Entry line that is a by-pass to the head of the regular immigration line. There is always an officer standing at the entry point insisting on seeing the passport card. Without the passport card, there is no way prove you have Global Entry. We have now started carrying the card in addition to our passports.

The Passport Card is also considered a TSA Real ID complaint identifaction for domestic air travel. Some states (e.g. Oregon are scheduled to issue Read ID complant drivers licenses just months before the 10/01/2020 deadline. Oregon is expected to charge an additional $20 for Real ID complaint DLs. We can of course use our Passport Books as Real IDs for domestic air travel or getting into a nuclear power plant.

https://www.tsa.gov/real-id

Beginning October 1, 2020, every air traveler 18 years of age and
older will need a REAL ID-compliant driver’s license, state-issued
enhanced driver’s license, or another acceptable form of ID to fly
within the United States.

https://www.tsa.gov/travel/security-screening/identification

Adult passengers 18 and over must show valid identification at the
airport checkpoint in order to travel.

Driver's licenses or other state photo identity cards issued by Department of Motor Vehicles (or equivalent)

U.S. passport

U.S. passport card

DHS trusted traveler cards (Global Entry, NEXUS, SENTRI, FAST) etc