1999 firebird alarm goes off shortly after arming with remote

[mechanicbmw]

we just got this 1999 firebird it is a 1 owner and was the pace car for the 1999 ric Pontiac 400 the car is in great shape but is showing some wear the alarm will sound shortly after locking doors with the remote

 

[melsg5]

Factory or aftermarket alarm and remote? If there are visible alarm switches check them. If there is a vibration sensor check it.