wierd single beep problem when i lock up

[unclepauly]

hello,

i have a boxster 2000, W Reg.

as the title suggests i have a wierd single beep problem when i lock up. however, it only happens when i open and close the boot (the rear one). it doesn't happen normally. in other words, this happens:

i get in the car and go for a drive. i get out and lock the car with the remote key fob. everything is fine. i unlock the car with the remote key fob, and then open the boot, and then close it. i then lock the car again. here i get the beep ! then i get back in the car and go for a drive, i get out and lock the car...its fine again !

can anyone explain this ? the only other thing i will say is that i have the car set on automatic locking, so the doors lock automatically when the car gets to 3mph. maybe the boot is only locking 'properly' when the car auto-locks itself, and not when i use the key. but this doesnt make sense ! and i was hoping someone could tell me why this is happening.