Havent been on here for a while, so sorry for late reply. I too had same problem this year on my 2003 S4R. At first I thought it was an oil leak as I had recently dropped my bike but then found out it was petrol from the tanks hinge. I looked into putting sealant in but someone had already done this and getting the old stuff out would be tricky. In the end I took it to a radiator/tank welders and they welded or braised it for £80. Not sure where you are but I'm up in Merseyside if you want there details I can pass them on to you. All the best.
Honda NC30, Suzuki SV650, Honda CBR1000F, Ducati Monster S4R