We found another bindrune hidden beneath the Prefects' bath in Ravenclaw. It doesn't mean anything; that antechamber is used for parties, anyone could know about it and potentially have had access. I'll let the headmaster know to speak to the Prefects when we all have more time.
The bindrune is currently on the wall below the window outside the Prefects' bath, hidden under a concealment charm. You should be able to reach it from the outside of the castle or inside, if you happen to be in Ravenclaw Tower. All the secret passages I know of have been sealed off; you would have to either break in or use the portrait hole.