I wonder if you can get a thermometer long enough to poke in there if it happens again... It would remove all doubt. It seems like it would be very possible to boil fuel, especially when you first open the cap. The B.P. of gas varies widely in range depending on octane and composition, but could be as low as 200 F. Given that other post about exhaust temperatures, and the fact that a few elises have lit up, I would check on it if it was me. I'm not saying there aren't other possibilities, but it seems like it's worth checking with a thermometer. I have not looked to see how shielded everything is in the back of the car, I've only had my front clam off, not the back.
Btw: did you actually SEE it bubbling out of the tank, big time? Was the smell of gas completely overpowering? Or did you just hear a hissing noise, like someone else suggested. I'd think if it was past the STP boiling point, you'd hear a pop or loud hiss the moment you opened the cap, and then smell lots of gas fumes and hear lots of bubbling.