What might cause water to leak under or behind a bathtub?
Your external list is pretty good, what's missing is an internal list: 


water shut off valves(stem leaks)
Connections between stop valves and mixing valve
Stem leaks on mixing valve
Shower arm and tub outlet extension: leaks at elbow inside wall


It is unusual for these sources to end up on the floor, outside the tub, unless your bath is over a slab.  

If you have access (if not, you should make one; 14" square centered on the valve) to the back of the mixing valve, use a flashlight and watch carefully when the water is first turned on, after its been off overnight