Someone would've AT LEAST had to have been notified, and even then I believe there are a bunch of people who would likely have a say. Otherwise, it's an unprovable thing. It's more likely imo that he's decided he declassified them conveniently after getting busted after presidency (convenient AND impossible to prove) than he truly declassified them during presidency without saying a word. Also, judging from what we've seen (you can play devil's advocate if you'd like, it's not infallible evidence) but I believe the clearly marked classified documents shown in a photo from Mar-a-lago would have to have been visually marked declassified on the front. Fun part is he made laws against this type of thing even tougher by bringing in laws against it to try and dunk on Hillary & her emails, iirc