I believe I sat next to one a few years ago - he didn't drink anything, he kept his jacket on all flight, despite it being warm, he was already in his seat when everyone else got on to board (I was at the front of the pre-boarding line, he was not in front of me), and he looked at everyone walking past as they boarded. His bag was stashed in the FA's locker (not the overhead). During the flight he didn't nap, didn't watch IFE, had a book but didn't seem to ever read it. I also wondered if he was put in the seat next to me to make it seem like we were a couple - there were a lot of men on this particular flight, so perhaps seating them next to a woman makes it seem more inconspicuous?