I would advise against asking a garage to investigate any intermittent problem on a vehicle, if at all possible. You might end up with a large bill and no fix.
Many vehicle alarm systems operate by using pressure sensors within the cabin that detect the pressure change as a result of opening a door or a window being broken.
It is possible that a cabin pressure change is caused by a passing vehicle or due to gusting wind. To minimise any such occurrences, when exiting the vehicle, ensure that all cabin air vents are in the close position and if fitted with aircon that it is set to recirculate rather than fresh air.