Hard to say without being hands-on with it, but I'd suspect a latch (the door ajar switch is integrated into the latch) or the alarm box itself (the alarms that are dealer installed). I've had some that went off due to the ajar switch and one that ended up being the alarm box itself that is "dealer installed" (local alarm dealer has a contract to install Ford-approved alarm system). Unlike every other Ford, it's hard to see what the car is seeing when the alarm does go off (no lock lights like the Fusions, no instrument cluster screen like every other Ford), so may need to have a dealer watch it with IDS for whatever is triggering it.