Well the fluid had to go some place.. So,,, if you can not find a leak on the line connections.. you could pull the slave cylinder and see if the fluid is leaking past the oil seal. Or,, the clutch plates are wearing enough to cause the slave cylinder to extend farther,, thus using more fluid.. But that is not likely. It would take a ton of wear... and it would be very gradual. Hardly noticeable. If your low on fluid,, no leaks at the lines,, check the slave cylinder