Bug in the system
Yesterday I took my Prius in to the dealer for a 15,000 mile checkup. Everything went fine, but when I went to pick it up and got in the car, a red triangle with an exclamation point appeared on the dashboard and the "check engine" light went on. Neither of these things were occurring before I brought the car in. I called a mechanic over. He sent someone out with a laptop kit that plugs directly into the car. That guy called over another guy. Soon there were 4 mechanics peering under the hood and poking around. 20 minutes later, problem solved: a gnat had flown in when the air filter was being changed, and lodged itself against a sensor.