things that go beep in the night and then some.

[Colleen F.]

Ok so I am stumped. I have a tenant complaining of a periodic beep in a 5 unit building. I can hear it, other tenants hear it occasionally but she hears it night and day every 15 to 45 minutes for like 1 second. The building has a fire lite MS-4 fire alarm but neither the inspector or the manual report this as an alarm this unit makes and the panel is green lighted. The emergency lights don't have a beeper and we have stood in front of the Verizon Fios boxes, septic alarm, Battery CO alarm and it wasn't coming from them. It doesn't seem to be in her apartment and when you sit in the front hall it seems to be referred to the upstairs but when you go upstairs it seems to be below you. What else can beep like this? None of the other tenants complain so I don't think it is coming from their apartments and she says it hasn't happened before. We are relatively new owners and put in a call to the old owner but I am open to suggestions.