I had a similar problem at my Mum's house, except that it wasn't.

She asked me to replace the alarm batteries in all her smoke alarms as she didn't want to climb to them. I did so despite being pretty sure there was life in some of them, but the beeps continued. Eventually by listening for the tiny beep that had faded to about once per five minutes I traced it to her (new to her) mobile. It was on PAYG, but when it beeped she had thought it needed more money, rather than recharging, it took her quite a while to use up the credit she had built up.

So...have you put away some other rechargeable device that might be signalling its distress?