ABS & Brake warning lights

2003 Focus SE wagon (US), 2.0L DOHC 16V ZETEC engine, automatic, optional anti-lock braking system, 117K miles. I don't ordinarily drive this vehicle during snow but last week I had an appointment and had to drive to a nearby city in a snowstorm. Most of the roads had not been plowed so I was driving on several inches of new snow. I parked the car in a snow covered lot for about an hour and then started the car to drive it home. The ABS & Brake warning lights stayed on after the startup light check. I drove the car about 5 miles (with both lights on) and stopped at the Post Office. Brakes seemed to be 100% operational although I did not get in a situation where the ABS activated. When I restarted the car, only the ABS stayed on. After driving about 1/8 mile, the ABS light went out. The car has been operating normally (no warning lights on) since then. I checked the brake fluid reservoir and it was full. No visible fluid leakage on garage floor. I don't think I have a problem but any ideas as to what might have cause the warning lights. Snow/ice buildup on rotors/calipers? It is possible that ABS was activated before I parked the car because it was slippery on a couple of downhills I encountered.