2007 Nissan Murano Alarm nightmare

During a cold spell, my Murano's battery went all the way dead. It jumped easily. However - beginning that evening, my alarm would go off in my car repeatedly for no reason. About every 15-30 minutes. I read it could be the battery, so I got a new one. I thought the problem was solved - and it was - for two days. Then it started again! I cannot even manually lock the doors - the alarm will go off randomly all day/night. If I have ANY door unlocked, it happens. All my doors are in sound condition, no shorts - tailgate is closed properly. I've did the "reset" thing with my key fob repeatedly - didn't change anything. I disconnected and reconnected the battery - that didn't do anything. My vehicle does have an aftermarket auto-start system installed. It still work perfectly. I tried using it one day instead of "normal" starting - the alarm still went off after I stopped driving. I've finally just emptied out my car & am forced to leave it unlocked - but that's a bad answer! I simply cannot afford to go to the dealership.