There's an electric pump that pressure tests the system for leaks whenever the car "wakes up". You may hear it cycle quickly when you open the door first thing in the morning.
I believe I posted a pic of this pump at one point. It's located inside the passenger side rear wheel well under the plastic shield. Its called a DTML Pump. Worth about $100 and takes about an hour start to finish. Easy job.
http://www.e90post.com/forums/showthread.php?t=260065
If its not a loose gas cap this would be my bet.