This is probably cause by you not putting the gas cap on tight enough, be sure you tighten it until it clicks. . If that does not fix it you may need a new sensor and wiring for it. Fix both of these issues and your problems should go away.
Check if the cap is completely screwed on tight with a couple of clicks. If not, it could be the sensor that tells if the cap is on or not. If you are handy with cars you could try to fix it, but i recommend taking it to the dealer.
You could have a faulty gas cap. If you have checked to make sure that your gas cap is on tight, then replace the gas cap. Most gas caps have a valve on them, like a pcv valve, if it is faulty it will cause problems.
You're losing vapor pressure somewhere in your car's fuel system. It could be anything back there from the gas cap to the tank. It's covered under your emissions system warranty.
