Temperature & Fuel Gauges On When Ignition Off

Hi, I have a 2002 Focus Estate, 2.0 Zetec, which has developed an odd fault. When I switch the ignition off, the Fuel & Temperature gauges go to
the halfway position (normally they park at zero) When the ignition is on, the gauges return to zero then work normally. This only started to happen yesterday, and the only clues I have is that I'd just filled up with petrol and then washed the car. Also, it's very cold and damp outside. I'm guessing some sort of electrical short-circuit and checked both fuse boxes, but they appear to be bone dry. I'd appreciate any advice! Cheers, Eddie
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