CEL turns on (and off) with no other symptoms
 

We have a (2004 or so) Protege5 which has been giving us no problems at all, until recently, when after about an hour of driving (in BC, so outside temp around 5-10 celsius these days), the check engine light turns on. The next day, it's off, and not producing any error codes by the time the mechanics look at it.

It's happened about 3-4 times so far, and of course we're never near a mechanic or during shop hours when it happens. Most recently it was on at the beginning of the day and then turned off before my wife could make it to the mechanic.

There have been no other symptoms (no rough idling or any other symptoms that I've seen mentioned in forums).

Until we catch it in the act, any suggestions for what it could be?