It's not just the change in atmospheric temperature Rozboon - the temperature-induced pressure difference also occurs each time you put the boat in to cold water on a hot day, and vice versa.
The pressure variance is proportional to the change in absolute temperature, so if you are looking at a 15 degree Celsius difference from say 25 degree air to 10 degree water, that becomes 298 absolute down to 283 absolute. That's only about 5%, so the pressure change would be +/- 0.05 bar, or +/- 0.7psi, which is pretty insignificant when you consider these sealed buoyancy tanks are pressure tested at the factory to ensure they are airtight. If the boat is sitting in the sun and out of the breeze though, the alloy can become skin-burning hot - maybe 60 degrees C - which could see a pressure change of more like 15%.
I have the same sort of sealed underfloor tanks, and only remove the bungs perhaps once a year, just to check that no water has got in.
It can sound a bit dramatic as the hot/cold air rushes in/out past the bung. Seeing a noticeable bulge or dip in the floor sounds a bit odd though - might be worth a quick call to the manufacturer in case it has become detached from the deck stringers.