'98 Escort Car Alarm Problem

I have 1998 Ford Escort SE with the 2.0L engine and about 90,000 miles. When the car is parked and locked, the alarm will go off every now and then for no apparent reason during the day while I'm in the office. The parking lot where it happens is quiet and weather doesn't seem to be a factor either.

Does anyone have an idea where to start looking for a fix? At first I thought it was due to a loose battery terminal connection. I've cleaned and tightened both terminals recently and the problem persists.

Thanks in advance for any suggestions,

Mike