Need help! Alarm kept going off!

I need help! So my alarm kept going of on my Audi A3 2004... I went out to make sure all the doors, windows, and so on were shut... as I got to the boot, and opened it I see that some how a bit of plastic has come off! After playing around with it and wedging a cloth in there, it finally said it's shut... and alarm hasn't gone of for a good 45min now 😂👍it's been raining really hard, and I wondered if some how the rain has got in there?? There is a bit of trim on the inside of the boot (near where this bit snapped off) that looks slightly bent.  I don't even know what the part is called, please look at the picture. Thank you!!