Loose Gas Cap - Malufunction

I have a 2008 Prius with about 34,000 miles. While I was driving yesterday the malfunction indicator lamp came on. I returned home (was 20 miles away), but did not notice anything wrong while driving. Thanks to Priuschat, I found discussions that said to check if the gas cap was on correctly; sure enough, mine was not on tightly. I've just put the cap back on, but when I started the car, the malfunction indicator still came on. Will the warning light clear on it's own (if the problem was the gas cap) or do I need to take it to the dealer?

 

Thanks very much; I'll put it through a few cycles and hope I don't have to visit the dealer! I've learned a lot reading this forum (it's a much better manual than the one that comes with car).