I'm totally prepared to hear that I'm the odd one here for being creeped out!
I was travelling by train for work with a colleague but our tickets were booked separately so we were in different carriages: me in C and him in D. I put my headphones in and closed my eyes. We'd been going about 90 minutes when I looked behind me to see how busy the train was and he'd moved seats to behind me but across the aisle. He'd moved about 15 minutes after we'd set off but hadn't said anything to me that whole time!
I do find him a little strange anyway so that may be colouring my view but aibu to find it odd that he just sat where he could watch me for the best part of an hour and a half without letting me know he was there?