Thanks for the responses, everyone.

Do the travelers have to provide the information, or do the airlines? And how would I go do this? (Note to self: go to post office and get your mail held as well.)

Oh yeah, I live in the US, and the rule is evidently for non-Schengen countries. I was looking at Spanish travel/tourism websites and haven't found any information about whether they collect the information.

Plus, I've never notified the embassy when I'm in another country, oops.

Edited to add: Okay, the airlines do it so I don't have to do anything. I found it an on Irish website, of all places.