Perhaps you are paying for another vehicle plugin. You should check your meter in the winter with all your electrical items shut off ans see if it is still running. Lots of sloppy electrical work in this town.
Good thinking, however, if there was another outside plug tied into my panel it would have to be mislabeled. Since I turn off the one marked for my plug ( and I've checked it by the light in my extension cord), if there is another one on that circuit, it gets turned off.