Problem with electronics?

I own a Vauxhall Astra 05 1.7 Diesel Mk5/H. I've had the car for over a year now and it has never done this before: when I put the key in the ignition (and do not switch the engine on), it makes a noise as though I have left the door open or the lights on (when I haven't). It's a beeping noise.

Does anyone know how I can stop this from happening?

Adam
February 2013