massive intermittent fuel leak

Hi All,
Had an unusual event occur.
Massive fuel leak that disappears.

It's happened twice, roll the bike out of the garage after a few weeks of not running, park on side stand,
Kit up and hit the starter, fuel pours out like its an open fuel pipe.

It appears to come from off the top of the block & streams down the casing on the left as that's the lean.

Turn bike off, put up on lift sitting level, pull side cover by tank.
Get the torch out glasses on & hit the starter.

Not a leak in sight....nothing, like it never happened.

Put bike on side stand again, start up and nothing...gone.

Any thoughts?

Pump seal? Remove tank and investigate.

In the uk at least petrol causes seals to swell. Then if not used for a while they shrink and leak but then when used have swollen again and don’t leak.