I left mine open when I was in the flat, worst thing that happened to me during freshers was my flatmate sellotaping bananas to my wardrobe and writing "KEANE" in huge letters on my wall in post-it notes (I'm not a fan of Keane). It's so weird going into other peoples flats and everyone always has their door shut, most of us have ours open when we're in the flat. Well, the girls do, anyway. Facebook/twitter raped my flatmate last night (instead of revising...woops) because he foolishly left his twitter logged on. Now very paranoid about making sure I'm not logged in because revenge will be swift. I should really lock mine more often, especially since it's a total state and I'd never notice if something went missing anyway. Our main door's always open and the door to the building locks automatically but if you press the buzzer someone will always answer it because we hate the noise. I always lock it when I leave the building, but if I'm going to the kitchen I'll leave it open, I close the door when I'm getting changed and stuff but don't lock it because my flatmates always knock.