I noticed a bit of a strange one this afternoon. I've got one of those wee cigarette socket voltage displays and it sits at 14.1-14.2 volts when the engine is running. But when the front parking sensors start beeping it drops to 12.5V. Doesn't happen with the rear ones and so far nothing else electrical in the car causes a drop. The most that will happen is 0.1V. Any thoughts as to what's happening there?