My phone infrequently thinks I've left home without actually moving - far enough to trigger the 400m geofence around it and to switch the webcam into "I've gone out, HIGH ALERT TIME!"

I assume it's because it's not getting good GPS (I'm inside a block of flats), there's no cellular signal, and therefore latches onto a visible WIFI SSID (I can see 15+) that it's got a location for which just happens to be [somewhere else].

(Similar to when my phone used to think Euston Station was in Manchester because it was picking up the SSIDs from the trains as part of AGPS.)