Gas cap of Doom

The other Night at about 6:30,the dashboard of my car issued an ominous warning: Loose. Gas. Cap. I parked the car and examined the gas cap;it looked okay to me, but what do I know? I got back in the car and Again saw the warning: LOOSE, GAS! CAP! So I took it to Sheridan Nissan, even though I knew the service department was closed, in the hope that someone who was hanging around there would know how to fix it. The service manager happened to be there. There was nothing wrong with the gas cap. It was tight. He unscrewed it and tightened it again. I started The car: LOOSE! GAS! CAP! he told me to come back tomorrow and they would fix it.i drove home with the message flashing at me: LOOSE GAS CAP!!!!!. The next day the message went away, permanently.