Because someone set off a trip wire, very recently,> she said. <There’s a fléchette stuck in the wood over there and a decent amount of fresh blood on the ground. That’s why there are three trip wires, not four. My guess is they followed Sark here, waited a bit too long to follow him in, and lost some flesh for it. But they must have got him, eventually.
It's disheartening to know the author doesn't trust the reader to figure this out from the prior environmental descriptions, so the added this "explain it to the clueless sidekick" speech.
