Check the rubber hose, going to the metal line. Check the clutch master cylinder, if its leaking, a majority of the time it'll leak inside of your car. If you have a worn out clutch, it could also cause low fluid because it is using the fluid more to compensate for loss of pressure/clamping force. Kinda the same way with brakes.

Check the line going to the slave cylinder, it should be an easy leak to find if it is leaking. Only a few places it can leak from.