First, it may not be actual GPS, but rather geofencing. Second, you should check to see what app is using it, regardless of gps/geofencing.
Go to settings, then click on Privacy. Select Location Services. At the very bottom, it shows you the different types of icons for gps v. Geofencing. In this lis, you can also see which app has used or is using the service.
This should give you a better idea of what is happening and why.