Do you lock with your electronic alarm or key? I had a customer once that had this issue and called back and said they just keep the trunk unlocked and it went away. To clarify on that last sentence you just have to push in the button to open the trunk not actually put in the key unlock and push.
2001 BMW Z3
3.0i straight 6 engine
M badge series
Dinan cold air intake, Dinan software, Dinan High Flow Throttle Body, Dinan stage 2 suspension upgrade, Dinan Strut Tower Brace, 18 inch wheels, and don't forget the fancy pedals.