It takes brake fluid.
When did this start? Any leaks on the driveway? On the firewall behind the Clutch Master? Any leaks up under the dash above the accelerator? You say it's not empty so I am guessing it's not full????
Bunch of scenarios here. Could be a small leak in a line, could be a corroded bleeder valve coming out of your slave, could be it just sucked up a bit of air if it's "not empty" but not full. Start with the simple first, check for some brake fluid leaks