2WD, AWD? You might need a new gas tank (a bummer is AWD, easy if 2WD) but you're also under some fairly unusual circumstances too like high temps with very low atmospheric pressure. I assume you're in CO? Before guessing further, I'd ask your dealer to call the tech hot line or their service rep to see if any of those guys might have heard of that and hopefully have a solution as gas leaking anywhere at anytime is not good. The device that's leaking is your carbon canistor and I've heard that it can become gas saturated (maybe even leak) IF the tank is overfilled (like fully topped up) BUT you shouldn't have to underfill it always to be safe either.