Dropping this here as speculation and hopefully for those with better internet resaech skills than myself to follow.
This morning on my to work, as I normally do I was listening to the news, and the ongoing reporting of the search of Mar-a-lago came up. In considering what documents Trump would want to squirl away, what documents he would consider "My top secret documents", a little Christmas tree lightbulb popped on in the back of my head. Wasn't there a Trump scandal, buried in all the many Trump scandals, where he was purposefully mis-classifying documents that could be damaging to him personally?
I'm now at work now, so my ability to research is very limited. A general google search is full of the current story, a search of Emptywheel found this THE INTELLIGENCE ISSUES THE HOUSE INTELLIGENCE COMMITTEE LARGELY IGNORED from 2019.
I don't think even Trump is dumb enough to try to outright sell state secrets, and if he had been caught with intelligence worth selling, the smart thing to do would have been to would have been to feign ignorance and give them back. But information damage to Trump... that I imagine he would fight tooth and nail to keep locked away.
I don't know that we ever found out what Trump was mis-classifying, but at least part of it seems to include his interactions with Russia and Ukraine.