Yep I had this. It's because rain gets in past the flap seal as it gets older. I put a large plastic lid on when it is parked out the back, no trouble since.
Your real problem is that the water that gets in will corrode the tank
I suggest anyone with a pregen take the invisible internal screw out now whilst it opens and carry an allen key in the tool tray that way it can never ever leave you stranded.