Was the tank full, 1/2, 1/4? Gasoline contains many things, including volatile fluids, like butane and propane. This will come out of it when the tank is shaken or warmed. Your tank has a vent to release any pressure, and to allow air in when you use the fuel.
Try a search for "07 ninja 650r fuel tank pressure" and you will find many discussions of this.
Try a search for "07 ninja 650r fuel tank pressure" and you will find many discussions of this.