Well it doesn't evaporate, so it's leaking somewhere. Perhaps out the piston inside the caliper...making it harder to see? Since you don't see any obvious leaks at any fittings, or your caliper hoses?
a booster doesn't hold fluid...so no
the only place on the res it can leak is out the fittings on the side, or the top cover seal. again it's got to be going somewhere. are you positive the front res is the front brakes?
