Soooo many people can get into flat blocks mate, easy to say now but should have taken it indoors. Quite a few of the main doors will have magnetic locks (easy to force open) or a route of access other than the main door. Also one of the neighbours could have had guests that have taken it, or someone could have let someone in... when you go out the main door, and someone is waiting to come in, do you slam the door in their face as they should have their own keycard, or do you just hold the door for them... definately not a secure place to leave an expensive bike! Either way, report it to the police. Someone may get pissed off with it having a piddly little gear to ride in and dump it after five minutes. Hope it turns up mate Dan