Well, I finished the game and again I heard 'beep beep'. And it didn't really come from the computer either. It came from beneath all my papers still waiting to be filed/thrown out. But papers don't beep beep. At least, not as far as I am aware of! Again I heard the 'beep beep' and this time I dug around a bit better.
It was my cell phone. The battery is nearly empty. 'Beep beep'!