I ran into this issue a couple of times in the last few days. I am traveling in a foreign country (Peru) with expensive call rates and no data plan, so I went into airplane mode to save some batterry. As soon as I was connected to the hotel Wifi the phone got my location.
This behavior can be explained very well by the Assisted GPS. I will keep an eye on that.
Do you know whether the cell towers provide this information even if I disabled mobile data?