Where are the FOBs sitting when this happens? I've accidentally hit the panic button a few times while FOBs were in my pockets or hanging on my belt. FOB means "F"ound "O"n "B"ottom. Sometimes Women throw them in there purse and it finds itself on the bottom with something else on top pressing the buttons randomly.
Why do you lock your vehicle if it's in your garage? Alarm should only go "OFF" if vehicle is locked. If vehicle is unlock then it is the panic that is being set off. Is your garage an attached garage? What condition are the buttons on your FOB?
If it's not the FOBs it has to be set off by interference. Either you have an FM radio station close by that transmits @ 104.9 MHz or 105.1MHz and produces a third harmonic strong enough to set off the panic alarm or you have a faulty RCDLR module. FOBs operate at 315 MHz. Do you have the 2 original FOBs? If so take the battery out of the one you normally use and use the other FOB. If the alarm sets off again you will have eliminated the FOB without the battery. Then reverse the process with the other FOB. Once you have eliminated the FOBs as the possible cause you can move on to other possible causes.