What can set your alarm off? Any one of the the door contacts can be faulty the same with the ultrasonic movement detector/s, a sudden voltage drop will also set off the alarm.

Vehicle alarm systems are not monitored so there's a bit of an issue with fault finding although u can isolate the ultrasonics buy locking and setting with the key which are known to become iffy, but still using the fob to unlock.

But then u may have to live with the problem as I did with my old Accord company car the alarm went off after a run in the cold weather for the last three years, temperature changes maybe, even the Main Dealer couldn't find the problem, lucky the car has now been changed so silence.

Might be a daft question, but Have you checked you haven't trapped a wire to one of the sensors when you replaced the roof lining ?