MM,
Two questions come to mind. First is it a real indication? ie does the fuel used plus the fuel in tanks equal the fuel on board at start. If this sum comes out positive then it is an indication problem and the temperature compensators are not working properly therefore giving an over-read of the quantity.
If the sum is equal then second question would be (not knowing the MD80) does the fuel cool the generator or oil system in the engine? If this is true and the fuel is recirculated to the tanks this could be slowly increasing the tank contents.