| sounds very odd. My st4s has never done this, but will regularly leave a puddle from the vent if I overfill it.
I'd be checking the pair of filler vents.
There is one which vents from around the outside of the filler neck. The other one vents from the top of the tank. Its unlikely this has dropped off else you would get fuel all over the place even when the tank wasn't full as the pressure builds up inside it (ask me how I know <roll eyes>).
The best way to check all the internal tank pipes is to remove the fuel pump & sender unit out of the bottom of the tank. You obviously need to drain it to do this. Unbolting the filler and lock won't show you anything. | 
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