My car does the same, I’ve never routinely removed the cap just to see, only when I need petrol and I generally brim full the tank and refill when near empty.
I think I’ll start opening the tank before and after journeys to find out what makes a difference, it can’t be good for it.
Re. MOT, cap security has been a standard part of the test for quite a while, it has to fasten on and seal.
