What a clever idea. Reminds me of the marks and signs hobos and other traveling people would leave on mailbox posts and sides of houses that would warn other vagabonds of dangers/hospitality to be found inside.
I can see sprinkling this into my game as a DM. PCs encounter one of these signs, and only the rogue has a clue as to what it means! (Wlll he tell the others, or keep it to himself?)