My suspicion is that you might have air in the system, and increasing temps causes it to expand, which reduces the free play, and even starts to disengage the clutch. I'd suggest a good flush before you start taking things apart.
I'm having the opposite problem - increasing freeplay to the point where the clutch drags a bit causing balky shifting. I can get more travel by pumping the pedal a few times. I suspect that my slave is on the fritz as I also spotted a trace of brake fluid nearby.
Despite bienniel flushings the slave won't last forever. Because both the master and slave are moved full stroke every time you declutch, the cup rubbers eventually wear. In a brake system there is little movement, especially at the calipers, so there is less wear over time.
I suspect I will be replacing or rebuilding my clutch slave cylinder in the near future.