Hi mate, this happens to my girlfriends car and also lots of other peoples on this forum. Its a faulty diode in the headunit which causes the unit to switch itself on randomly, even with no key in the ignition! Seat used to send the units away for repair for about £150 i believe but then some members on here went to the repairer direct and got the cost down to £75. Unfortunately the place has now stopped repairing the units (the guy who used to do it left the company), now the only thing seat can do is offer you a replacement unit for £400!!!!
My girlfriend just removes the fuse at the moment to stop her battery going flat, however i have thought of a simple way to add a switch inline with the circuit, ill just look for the thread for you......