Well if it’s something like a turn radius, they can always claim that they just guessed right.
Are they under any obligation to protect the classified information if they’re not the ones who leaked it?
No. You can publish it if you like. This is how journalists work. You cannot get someone to commit a crime towards getting classified documents (Assange tried to teach people to hack shit and pled guilty to this). But accepting them and publishing them is fine and good.
Knowingly owning or using classified info without proper clearance is, in fact, a crime.
That’s a large part of what Trump’s classified document raid was for. Former presidents usually have a lot of classified stuff to turn over after leaving office. It’s standard practice, (and perfectly legal) to simply send it back (via the proper channels) as soon as you discover you have it. But if you conceal it and refuse to return it (like Trump did) then that’ll land you in some hot water.
No, his problem is that he was supposed to have access at some point wherein you agree to secure documents in certain ways. If you’re given it or find it you do not have that duty. Defense secrets are much more protected but still not that much of you aren’t supposed to have access.
It’s my understanding that you’re only required to protect the information if you’ve actually agreed to do so, which is obviously a retirement for being given access. Elected officials are a weird area where they have a much easier time getting clearance, but they’ve still made agreements to protect the information.
Trump was authorized to handle classified information in the first place, which is why his mishandling was a problem. I haven’t read the actual law, but I’m pretty sure ordinary people who happen across classified information have no duty at all in any direction. If you can show me an example of a random person getting in trouble for sharing classified information that they didn’t steal or get others to steal, well, let me know.