It shouldn't happen and just unplugging the power cable sholdn't have fixed any known problems unless you then had a dead battery
First suspect is the thermostat. Plug everything back in and, the next time the fan runs when it shouldn't,, disconnect one of the wires from the thermostat. If it stops after a few minutes you may have a bad thermostat. If it doesn't stopyou have a wiring troubleshooting problem of a circuit board problem to resolve. Remember, it takes a few minutes for the fan to shut down when the thermostate is turned off.
BTW: If the LP gas was OFF at the tank the fan would still run but the furnace