Alarm playing up

Hi there, I hope someone might be able to help. A friend of mine over here in Portugal has a Discovery 300 Tdi and the few days he has been having nothing but trouble with his alarm system.

It takes him ages to manage to get it to lock and activate the system and then, after some time the alarm decides to fire.

He started to think that the fob's battery was on it's way out, but when the alarm started firing without out any external influence he realized it could be something other than the fob.

Does anyone have a clue what it might be?

Thanks in advance.

PS: It is a Disco I and not a II as I chose by mistake.

Happend tome once it was a big bee in the car

Lock the vehicle with the key, that will isolate the internal ultrasonic movement detector, or even pull apart its connector plug and see if the problem goes away..
Remember to unlock with the fob.