I guess I don't fully understand what happened or how you lost the flash drive. You gave it to him and then went to the bathroom? Then he gave it back after and it was empty?

There is nothing really to add here that someone else hasn't already said.

Essentially, you're crazy if you go try to sell crypto to a complete stranger. Then leave said stranger with a flash drive containing your btc while you go to the bathroom.

This all seems like common sense. I do feel bad for you though.