Oh, they're for daemon summoning. I canceled them out, see - [He points to the slashes through all of them.] - so they're not active right now. Be calling up some pretty nasty stuff if they were.

[spoilers: it's him, he is the daemon that these particular sigils call, and he doesn't want his metaphysical phone number written down for just any rando to copy]

Still, seems like a bad thing to leave lying around. I don't want kids thinking this is the cool new graffiti trend.