My guess is you have a bad check valve. Water is bleeding back so slowly that it is not spinning the meter backwards. But when the pump starts, the water is coming fast enough to spin the meter forward. So it looks like you are using water even though you are putting it right back into the city main. Time of year or temperature has nothing to do with it. If you are sure there is no leak, back through the check valve is the only other place water could be going.