I have observed this as well. Plenty of fuel in the top tank - enough to see the fuel level. The surface was disturbed and it appeared to be caused by bubbles, which I assumed were coming from the lower tank.
The smell of petrol was coming from on of the rubber tubes that emerge below the bottom of the rear of the crankcase behind the large footpeg bracket near to the gear lever. This was fuel vapour - I could see the light shimmer / disturbance caused by the emerging fumes. Like a heat haze.
This has only happened when the engine is very hot and I've only ever noticed it on very hot days.
There is going to be considerable disturbance of fuel between the two tanks - hotter fuel will rise, colder fuel will sink, and without any fast air movement the temperature difference between the upper and lower tanks will change.
Any fuel next to the tank sides, closest to the engine / exhaust may turn into vapour, like you see in a pan of water as soon as you put it on the stove. Movement upwards through the surrounding fuel will cool most if them down again.
To boil, all of the fuel needs to be doing this, which isn't going to happen. But the behaviour is normal.
What is more alarming is the ST1300 ghostly cry - like opening your mouth and making a sound while breathing in. It happens at night when you go into the garage alone, hours after a long ride. The bike has cooled but there is still that really pleasant smell of warm bike, oil and fuel and that alone may be the reason for your nocturnal visit. I'm sure that it is addictive
You've just settled in to carrying out a few aimless tasks - to justify popping in for a 'fix'. So you're probably moving things around from where you last put them, to somewhere else where you won't be able to find them (like their proper place). Whatever it is, you are engrossed and quite happy with the fact that you are alone in there in the middle if the night.
Then without warning, there is this deep, hollow, ghostly, 'last gasp' moan which instantly raises the hackles on the back of your neck
What the .....