2011 A3 Cab 2.0 TDi alarm malfunction

Hi - relative newby here. I bought my lovely A3 in August and other than having a regular battery fitted so the auto stop/start function doesn't work (for which I got a deal on wheel refurb), it has been trouble free.

Recently the battery has been draining to the point I thought the battery was failing, but a basic check has come out ok. I then discovered the alarm was going off. I read that as the battery discharges, various functions shut down to preserve the battery so I think that may have happened with the alarm however recently, when  park up at home, within an hour the alarm goes off. I unlock the car and lock it but the alarm goes off again - most times. I've opened doors and then re-locked, but later the hazards come on. I suspect this is why my battery is draining. I thought maybe a spider was in the vehicle and walking over the sensors so I've put a cloth over the dash, but no different. Tonight I read the handbook and checked the diode response. When I lock the car remotely, the diode flashes six times then stays on for 25 seconds or so then flashes normally. The handbook says if it stays on for 30 seconds there is a malfunction. Seems clear to me there is! Anyone got a fix? The car was bought from an independent and is under warranty, but they are unlikely to be able to fix something like this, so I need info before I go back to them. 

Cheers!