Unless you are in the same room, I don't see how. It is possible that someone could set up a hotspot using your mobile connection and tether to your phone if they are physically close, but that's pretty unlikely. First of all, most carriers (in the US anyway) block tethering unless you pay an extra monthly charge. Second, there would be an icon on the notification bar at the top of your screen.
If you are seeing unexpectedly high data use, the likely culprit is an app.