I'm reposting this lower and making some edits so that it's sequentially after David's latest set of information. (Thanks, David!)
Hey folks, I've been asked to review this thread. There's limited information here to indicate what this actually is. Please don't jump to conclusions that there's been some sort of hack based on such limited information. I've edited a post in this thread to tone this down as online alarmism isn't helpful.
For another example, claims of "I only use X password on Y website" seem reasonable to assert that Y website has a problem on the surface. However, there are a number of other ways beyond an issue with Y website to acquire such a password. Your browser can remember passwords. People write down their password in other apps or email them to themselves. Your device may have malware on it that tracks keystrokes (aka tracks username/passwords), or perhaps you logged in once on a other/public device that has such malware. Or, as David mentioned, perhaps the password just isn't that strong. And I'm not claiming that anyone here specifically is forgetful, but I've had people on the phone and in other conversations say something like "oh yeah, I have used that password somewhere else." Frankly, unless you log in with a custom email address that only is used to log in here (usually requires running your own servers to create a e.g. ricksteves-login @ MyCustomServerAndDomain.net email address) -- and even then it doesn't rule out the possibilities above -- we're in a position to take such assessments (using X password on Y website) with a grain of salt. That's not to say that we don't take e.g. David's experience seriously as it's worth investigating.
No one needs to become overly alarmed about the possibilities I've outlined above either. The best advice is to simply update your password if you're concerned.
For those who understand data breaches and how searching the dark web for compromised usernames/passwords works, in case it eases your concerns, I'm not seeing ricksteves.com on the https://haveibeenpwned.com website noted above. That's not foolproof and that service may not be comprehensive, but it shows that our website hasn't been a part of known data breaches. There are other services that provide similar information that can help indicate where a breach came from as it relates to your data. By all means, if you ever saw ricksteves.com, please let me know directly instead of posting in a forum where I may not see it! And in case it isn't obvious at this point, this reply indicates that to our best knowledge RSE has not been involved in any breaches regarding your RS Account.
I'm happy to review your specific case in case there is a sniff of a problem on our side. As much as I'm trying to temper gut reactions in this thread -- especially as Apple/Google/Msft/Safari/Chrome/Edge/Firefox step up their efforts to notify you of compromised logins -- I do take security issues seriously (as does the rest of our IT team and management). If you'd like to send me a screenshot of what you're seeing -- and make sure the screenshot includes what web page you're on including the URL at the top plus the entirety of the message/window that pops up (do your best if on your phone) -- you may do so at webmaster at ricksteves dot com. Please provide any other specific context around what you were doing at the time you saw the message.
Thanks everyone!