Gas Leak...

Doesn't look like petrol but more like brake fluid although it can't be that as it's on the wrong side. Long term battery acid looks similar.

 

Doesn't look like like petrol to me
Petrol wouldn't lift the paint like that and by the time it got to the floor most of it would evaporate and maybe leave a stain not a puddle
Looks more like brake or clutch fluid to me
Think you're gonna have to lift the tank to see where it's coming from

 

its brake fluid or battery acid . check your little fluid container on the right handle bar then your battery under the seat (seems to be alot though)
Your bolt needs lined up and you should take off the tank to see why its loose and that it has something to screw in to.
Have you seem the fluid coming out or did you find it like that?
If it is petrol then your tank fuel pipe was not secured, there a right angle clip on pipe that secures it this may have come loose you need to inspect this, what does it smell like
Acidic =battery
nothing = brake
petrol=petrol
Dont always believe garages in fact dont ever believe garages not completely .Is that the plastic tank model cos when you take the tank off with petrol in it the tank expands so the bolt wont fit it does shrink down though. Couple of Thruxtons on here stalling in traffic, I think it was a poor seal on the throttle bodies
Keep us informed

 

Must be brake fluid

 

The bolt that holds the fuel tank (at the front also seems to have paint stripped so I thought brake fluid

 

If it is petrol/gas then you have a leak, possibly near the front of the tank, but not necessarily so as fuel will spread quickly over a wide area. Although the paint is lifting at the front of the tank, it may be leaking elsewhere and spreading along there. Does the liquid actually smell like petrol/gas?
Can't see that it would be brake fluid as the reservoir is not that close to that part of the tank - unless it was spilt on there by someone else.
The battery is under the rider's seat, behind the fuel tank which seems to rule out a split battery, but if they have been messing about in that area and the battery has been removed, then acid from the breather pipe may have been spilled over the frame.
A lot of ifs, buts and maybes to contend with I'm afraid, including the tank swelling due to its reaction to ethanol in the petrol/gas. I think we are on 5% ethanol over here but I think it may be more in the USA (I'm sure I read it somewhere!), and tank swelling is not unheard of in the UK.
Obviously be careful with whatever liquid it is - petrol/gas for obvious reasons, battery acid will burn most things and brake fluid is an irritant if it gets on your skin.
Happy days!

 

I would say remove the tank and dry it off completely. Then wait to see where the leak appears to be. Remember though that fuel will spread quickly - something to do with a lack of surface tension i.e. it doesn't form globules or droplets to the same degree that water does, or a more extreme example such as mercury. A small pin prick or a slight weep will produce more evidence than you would normally expect.
I must admit that I am surprised by the damage done to the paintwork on the frame. Your petrol/gas must be pure sh1te!! That's assuming that is what caused it.

 

What year is it? when was the black frame introduced? looks like the frame has been sprayed with a satin black acrylic and you've got a petrol leak was it over filled and the fuel come out of the overflow (which may have the pipe missing/off) does it still leak now? fuel pump plate gasket?

 

Well you need to take the tank off and have a look. There's no point just beating about the bush!