If you didn't go to the toilet during, then they know for sure you didn't go on your phone during the toilet, though I don't know if they keep track of who goes to the toilet (and it depends if you did go to the toilet during). Personally I'd explain I left it there before, I mean you can't really get in trouble for leaving your phone somewhere else, the problem is if you have it in your pocket. Also I'd be wanting my phone back.