Not sure what this could be...?

I'm hoping someone can provide some answers on this. I'm actually a little creeped out by it... My husband bought a home years and years before I met him as an investment property. There have been various renters, but we just moved into the house two weeks ago. Built in 1998, so fairly new. Don't know if that's relevant or not.
So, ever since we moved in I've noticed that if you are standing in the master bathroom and someone opens or closes the back door, you can hear a bell go off in the attic. Not an electronic bell, just a bell. You can only hear it from the master bathroom. Honestly, it's pretty creepy. Maybe a renter installed it to catch their teenager sneaking out? I'm not sure what to think.
We're not able to get into the attic at the moment because the ladder is broken and not sturdy enough to climb. I'm hoping someone can shed some light on this.
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