Beep From Dash When Engine Is Off

[Ironbeard]

Yesterday, had a flat Battery as the lights were inadvertently left on during the day (yes, one my dumb moments but in my defense, first time it's ever happened). Got jump started, and drove back home. I then disconnected the Battery, put in on charge overnight to top the charge back up and reconnected the Battery to the car in the morning.

Now there's this periodic beep from the dash about every 3 seconds when ever the car is parked and the engine is off. Turn the ignition to the On position and this beep goes away nor is it present while driving. Engine appears to fire up first time so far without incident. As soon as I turn the motor off, the beeping returns. Nothing appears on the dash.

The beep sounds more like a cricket chirp (or the sound of an old casio watch that would notify you each hour at the top of the hour). My searching on Google hasn't dug up anything. Car is a 2006 (JDM) Toyota Avensis 2.0 vvti D4 wagon

Any ideas what this beep indicates?