[artart]

My guess would be master cylinder leaking by internally, if you are not losing fluid.
That should be relatively easy to detect (loss on the ground or under the car somewhere) if it's a leak outward it is going somewhere but may not make it to the ground ( line, hoses. etc. run everywhere) BUT master cylinder level has to show the loss of level unless it's sliding back and forth inside that unit.
This is all based on the premise that you haven't had any recent brake work done or refilled the MC reservour such that air could have got into the hydrallic system. If you or someone did, it could be air and require bleeding out. Then the question remains where did the fluid go, it is not consumed.