Where is the spot on the concrete in relation to the bike? Right around the swingarm pivot? Do the drain spigots on your carb bowls have tubes on them, and where do they run?
Do you turn the petcock off everytime you park? Sticky floats can cause fuel to overflow out the carb bowl drains, and if you leave your petcock open and/or if it doesn't seal perfectly when closed, fuel will flow into the bowls and overflow once you've shut the engine down and it's no longer consuming the fuel from the bowls.
Depending on where the bike is parked, the gas could evaporate pretty quickly, leaving a basketball sized (which is pretty darn big) spot of gasoline from a constant drip from a single carb bowl.
Also, do you park the bike with the side stand or centre stand? Leaning to the left might contribute to the problem if bowl #1 or #2 are the problem. Drop the bowls one at a time and check the gaskets and float operation. Check the valves too - with the petcock off, remove a bowl. Hold the float up with your finger, then turn on the petcock a bit with a rag under the carbs to soak up gas. No gas should drip. Lower the float, and at one point the valve will open and gas should flow.
If gas is flowing with the float in the upper position, the valve is stuck open and either needs cleaning (soak in carb cleaner) or replacing. If the float is sticky or stuck in the down position, the valve won't be closed as fuel fills the bowl and the float rises (same as in your toilet tank).