I just spoke to a third 2002 xstar owner who said that 'every now and then he'd hear pressure or vacuum escape from the tank as he unscrewed the gas cap, but it didn't last more than a second'...
May there be a one way check-valve that allows air to enter the tank to replace fuel that leaves the tank, but doesn't allow air (or fuel if the boat sinks) out of the tank?? Maybe the temperature rose during the day and the fuel and air expanded causing the tank to pressurize if a check-valve won't let anything out?..
Can I be sure the vent is literally an open line to the atmosphere on these boats? I also have a scarab, and on that boat the vent most certainly is an open line to atmosphere.