This website describes the phenomenon nicely.
Anyway, on Monday night, Dr. J.'s warning light for his brakes came on in the truck. So, yesterday morning, Dr. J.left early to get to the dealership before 7AM. Said dealership is about 20 miles away. 10 miles into the drive the warning light goes off. I made the journey anyway, as this is the second time this happened (everything checked out when it happened a few months ago).
So this time, the dealer said Dr. J.'s brakes are still fine, but the brake sensor (the part telling me the brakes have a problem) has a problem and needs to be replaced. Fortunately it's under warranty, but I have to come back as they have to order the part. Dr. J. longs for the good ole days when a warning light coming on didn't mean the sensor was broken, and if you were handy, you could fix it yourself, or at least take it to the local mechanic to fix.