In some cars with mystery brake fluid loss you also see leaking from the master cylinder into the brake booster. Look around the firewall for anything leaking as well, but I have not seen anyone on here with that problem.
I agree with Wagner above about the brake fliud level correlation to the pad thickness but I always keep the fluid topped off (and change it every year or so) and just vacuum some out of the reservior before compressing the pistons when changing pads.
2005 T&C Signature Series, 64,xxx miles
Lots of rust, leaking steering rack, bad rear AC condenser coil, rusted (and replaced) rear brake line. Looking forward to the next failure/defect that this Chrysler will bring into our lives.