Wireless? Does this happen all over, including further away from the city? You might be picking up a slow signal of some kind, enough to keep the time running but not enough to display speed. (I've never had a Sigma, though it probably handles interference better than the cheapo wireless computer I used to have.)

Otherwise, did you do this several times? If you stop at a light with the magnet right next to the sensor, normal little back and forth movements could do the same thing.