Iron flag on... nothing?

A Great Gate A large gate of heavy wood blocks the passage north. Great iron spikes have been hammered through the wood into the stone walls themselves. The gate and its supports extend from floor to ceiling in the passage, denying access to any who hope to breach the inner secrets of the Tribe of the Bleeding Wind. Several torches held in sconces light the area, though they seem of poor quality. Occasional shapes move about the other side of the gate, visible through small gaps in the wood. At the top of the gate is the depiction of a hydra burned into the wood. Exits: [north] south (Iron) I can't see anything there that it's flagging. And I have detect invis on at that point, too.