The bottle may burst if left in an extremely hot environment. Trust me.

A few weeks ago I had just got done with the most thorough cleaning I had ever done on my xterra. I actually removed the seats and the molded carpeting to clean the entire interior to make it like new. It was the cleanest it had been since I drove it off of the lot 5 years ago...

So the next day I throw my trucks and fuel and everything in the back so I can bash after work. I keep my small amounts of traveling nitro with me in traxxas tope-fuel quarts so I don't have to carry whole gallons around (top fuel quarts leftover from when I was new to nitro and still willing to pay for traxxas fuel). When I went out to my car after work I found everything in the rear of my car sprayed with drops of nitro. Here the quart heated up enough and pressure built up and it blew the plastic top right off of the bottle, not just the screw on cap, but the entire plastic part of the bottle where it comes out of the metal part of the quart container. Needless to say I was pissed, not just because of the mess, but because I know how nitro can dissolve synthetic fibers. Anyone whose ever bought an RC car bag made by trinity probably already know all about how nitro dissolves certain synthetic fibers. My almost new trinity maxx hauler bag is falling to pieces in every spot where I've ever spilled any nitro. My car on a hot day is surely hotter than the temps you will see on your sunporch, but hey who knows. Since then, I've found that the problem is no longer an issue as long as I leave the quart containers at most only 3/4 full when they are going to be in my car. That allows the gasses to expand without building up too much pressure.

The same could happen to gallons of fuel left in the sun if they are extremely full. They could build up pressure and blow the hard plastic screw on top off. It's easy enough to avoid this by squeezing excess air from your fuel jugs before you screw the top on, it allows for a bit more expansion without catastrophic results