Why would a 97 Altima horn honk and lights flash continuously like an alarm when you parked it and hit the power locks if you don't have an alarm?

Answer for crazy alarmmy 1997 Nissan altima is similar when i park the car and lock the dorrs its fine when i return to the car and open the door with the key the lights start flashing and the horn/alarm starts going off when i put the key in the ignission the alarm stops... very intresting and wierd......