Garage pir?

Hi,recently moved into our house that has an accenta alarm fitted with 4 pirs on the system,one of which is in the adjoining garage.

The one in the garage had been omitted from the system by the previous owners (I know why now ) as it activates the system now when set intermittently.

The pir was originally situated at the rear top corner of the garage looking at the double doors,so I moved it and placed it above the double doors looking at the rear of the garage.

Thought draughts or minor movement of the doors would be causing the problem,

darn thing still activating the alarm intermittently,i have removed and cleaned the pir and checked the wiring,i have even swapped the pir for one in the house.

Iam now at a loss of what to do,

coverage in the garage would be a great asset as I store expensive trade tools in there.

Can you help/suggest,have I missed something?,

I would be very grateful if you can and thanks.