I would posit that a person who plugs the USB key in leans more towards thief than "person who just wants to get the keys back to you." I'd further bet that the

I would posit that a person who plugs the USB key in leans more towards thief than "person who just wants to get the keys back to you." I'd further bet that there have been a not-insignificant number of CFAA prosecutions for actions of curiosity. (Not to say that plugging in a found USB key constitutes a CFAA violation)