Does it say a specific sensor, like basement window? You can typically replace the batteries yourself. You pry open the outer box lid carefully and replace it.
If you are new to the house, invite ADT out to provide you with a map of all sensors and to see if things work.

Just disconnect the wires from the battery. I had a similar issue and once the alarm went off in the middle of the night. That was fun.

I bought this house two years ago and it has an old ADT alarm system in it. I don't have any info for it, like passcodes. It's never been a problem but recently the front panel starts beeping. Hitting any button stops it. But it has done this four times today. I googled and see it's a battery in the basement. But the instructions for replacing it require passcodes to put it in test mode.

I can call ADT tomorrow but wondered if anyone here has been through this and has a better solution so we aren't up all night with repeated alarms. Thanks.