A.S.
Good morning!
We have an ADT alarm, and it does go wild sometimes. It sounds as if the battery may be going out.
Two thoughts. Radio Shack sells new batteries for $80 which would hopefully fix the problem. Or calling ADT and having them come and do a maintenance on the unit but not sign up for service; this way, you could have a "local" alarm that does emit an alarm and scare bad guys away. We did that and it cos $80 as well.
I am not sure how to disconnect the unit without all sorts of issues though.
Hope this helps and good luck.