I believe the stop lamp circuit is a constant battery circuit.  The brake lights light up with the key off right?  Anyway, the fuse is before the stop lamp switch in the circuit.  That means that there is battery power to the fuse without the brake lamps on.  It's called open circuit voltage.  Or, the voltage present with no loads on.  I think you need to find an ammeter and install that to check for drains.  A circuit can have voltage present, but that doesn't mean that there is any current flow.  It's the current flow(amps) that drains your battery.