There have been many top secret leaks on line, one game in particular has been the reason for a lot. The game War Thunder has had service personnel from many countries disclose secret info to back up their positions. Leaks have come from the US, England, China, Russia, France and other countries. Here is an interesting article. There are other articles out there.
Another problem is that according to one article I read, as of 2019 there were about 1.25 MILLION people holding Top Secret clearance in the US, seems like a lot.
War Thunder fan reportedly leaks restricted F-16 documents - AeroTime