Same but different scenario. The plumber was telling me that the Council were renovating this house. The renovations were all done and the tenant had been back in for a few days.
She kept hearing this beeping noise and it was driving her mad. Workman came back and checked the carbon monoxide detector, smoke alarms, etc. He stood and listened and said it sounded like a smoke alarm with a dead battery - beeping a warning for a new battery. He was downstairs but it sounded like it was coming from upstairs.
He went upstairs to listen for it and it sounded like it was coming from downstairs.
Do you know where I'm going here?
Eventually sussed out that somebody had kicked the old battery operated smoke alarm down the floorboards so it was between the floor upstairs and the ceiling downstairs........
Dunno how they sorted it though.