That stinks. How exactly was the bike locked up? That will give you an idea if it was a slow or fast job. Depending on how long it may have taken, it may give some insight to whether or not it was really an inside job. Either way, I would post a message in the courtyard to see if someone let anyone in by accident - since keeping the bike there was a sensitive issue with gas and all, you can say someone stole your bike with no description in the flier. There are only a few ways that someone could have nabbed that from a reasonably secure location...
1. It's another bicycle owner in the courtyard who took a fancy to the thing
2. It's a friend who visits often of someone who lives in the complex and asked someone to let them into the area
3. It's a person who visits often enough to know it's there, and was accidentally let in.
4. Someone notified the building management and/or complained about it and it was towed or something like that.
Despite the issue of having a moped parked, I'd ask the building management if they knew anything. Sounds more probable than a bunch of kids gaining entry to the courtyard just to steal one moped. You could say it was a friend who got caught in the rain and had to park the bike there or something like that if it will indemnify you of being the owner.
Just a note on locks if it helps give a timeline:
Cable-type locks - Mostly defeated in a matter of seconds with a proper cutter. Very easy for inexperienced thieves
Normal Chain locks - Mostly defeated in a matter of minutes with a proper pry bar or cutter
Generic or bicycle u-locks - Can be defeated in a matter of minutes, or rendered useless (key won't work anymore) in the process of theft
Motorcycle u-locks - Kryptonite and On Guard are not easily defeated as are similar models
Motorcycle chains - Kryptonite and On Guard chains are extremely difficult to defeat without more powerful or electrical tools.