The new epa-compliant tanks will hold pressure up to something between 5 and 7 PSI. They will swell visibly, but have been tested to hold this pressure. T
The pressure your wife saw might have seemed a lot, but may have been within the designed range. Still, it's not good practice to let the tank sit in the sun.
A vital part of the safety of these tanks is not over-filling them - they need some air space at the top of the tank.
Interesting link, apparently they will vent outwards at about 7 lbs, which seems like a lot? As mentioned in an earlier post, I modified my cap to vent both ways, which was a mistake as it appears the primer bulb doesn't work well without pressure in the tank?
If you don't want to be sprayed with gas when disconnecting the hose, just loosen the cap first. I just ordered a replacement cap for the cap I modified, about a $50 mistake.