Fuel Leak Overnight - Stuck Float Valve?

Yes, its a very likely cause and a very easy one to check and fix.

IMHO, it could be:  1) over-weight/sinking floats and/or 2) a malfunctioning float needle valve or valve assembly.  

Floats should weigh-in at 3.5 grams each or less.  Verify that there no binding between the floats and float bowl. Heavy floats are a likely cause of your problem.

It could be some grit or debris causing the needle valve to be cracked open. Inspect the condition of the float needle valve tip and verify it's spring action. Inspect the condition of the valve seat. Inspect the valve assembly for proper operation and alignment.  There is a procedure in the ROTAX Heavy Maintenance Manual (https://www.rotax-owner.com/manuals/MMH_912-914_Series_ED1_R6.pdf) page 32 regarding these items.  There is also a procedure for pressure testing the float needle valve for leak checking.

Although I don't have an empty tank my left carb is doing the same thing. Floats and etc were replaced recently. Ordered new needle valve close to 50 bucks. It does not look like the seat can be replaced. Fuel is coming out of the needle jet and running into the air filter then dripping on to cowling and then to the floor. Would not have even seen it if parked outside on soft surface.   

Question:  when was the last time you folks had your carbs properly inspected (to include disassembly)?  It really ought to be done every 200 hours.

A flooded carb will overflow through the carb jets into the carb throat.
If your engine sits anywhere near level in the hanger, it is possible that some amount of the fuel flooded the intake manifold and there is a 50% chance that when the engine stopped that one of the intake valves on that side stopped in the open position. 
The fuel would flow into the cylinder and, over the course of hours, slowly leaked through the gap in the piston rings in the crankcase.
It would be recommended that you check your Oil for fuel contamination.

"Burp" the crankcase as usual and then check the Oil Tank for the smell of fuel.
If the Oil Tank is found to be Above the "Full" mark,  You might be looking at 4 Quarts of Oil Plus a Pint of Gasoline.

Something to think about.