ml430 alarm sounding intermittently for no obvious reason

Have a 2000 reg ML430.  Experiencing issues with the alarm. Goes off intermittently for no obvious reason - seems to happen every few hours - even does it when driving.  Been having some issues with central locking but they have been fixed by a replacement switch being installed BUT this alarm problem has been going on for a while.  In the past we had found that the alarm would not sound if we locked the car using the central locking rather than the key fob, but this now seems to make no difference (since we had the central locking fixed that is). Lights dont flash when alarm sounds.  We are probably going to have to take back to the garage BUT wondered if there could be any obvious (and well known) causes -for example, does the battery need replacing on the alarm system? Is it a door sensor perhaps? Any suggestions would be helpful so either we can fix it OR we can at least point the garage in the right direction (and so save potentially on a lot of labour costs)