stolen Trek

There is a bike rack in the courtyard of our apartment building where 3-5 bikes are parked at any time. The courtyard is behind a gate, but the pneumatic gate hinge often causes the gate to not close. I had left my bike out leaning against the bike rack and had forgotten to lock it up that evening. I went to bed around midnight; my friend came home around 1:30am and the bikes were still there. I walked out at 9am and my bike was gone, along with two other bikes that had been locked up with u-locks. There has been on previous incidence of bike theft at this apartment complex that I know and it was 2 years ago; my bike seat was stolen, along with several wheels (not mine).