The blood is fresh, and if they follow the splatter to a second one, they'll soon see why. A large spider (not a giant spider, but about the size of a dog) lies dead nearby, on its back with its long legs curled up. The smell of iron wafts through the air around it and one can see it's cut open. The blood clearly belongs to it.

And where one spider is, there are usually others. The question is whether they're alive, or have been killed like this one. Merrill and Korrin can move on down a corridor nearby, or check the alcoves first.