A mobile device with a display and location services may store information pertaining to a geofence. With the aid of the location services of the mobile device, a geofence crossing may be detected when the mobile device crosses the stored geofence and, if a geofence crossing is detected, the mobile device may transmit a corresponding geofence crossing event to a remote location via a transmitter of the mobile device. The size of the geofence may be automatically adjusted when a predetermined abnormality is suspected in the detected geofence crossing events. In some cases, a query is provided to the user to verify the presence of the predetermined abnormality before the size of the geofence is automatically adjusted, or a user-initiated menu may be employed to verify the presence of the predetermined abnormality and/or to adjust the size of the geofence.