luggage compartment open buzzer

occasionally, say 30 seconds after starting up, i get the "luggage compartment open" message and buzzer.

then it wont do it again for the rest of the journey.

the doors and boot are all firmly shut, yet it still does it. i think the message also displays if a door is opened (ignition on obviously).

so what is making it do this intermittantly?

1.9TDi L&K hatchback