Swamped

Well. You still don't have an explanation of the noise, so you think a bit more carefully.

If it wasn't the children, could it have been whoever made that glowing symbol? Then why did they leave their knife in the floor?

Maybe they heard you coming and decided to hide. And maybe this symbol was important enough that they rushed to finish it first, but were in too much of a hurry to pull the knife out afterwards.

But there was a big crowd of officers here just a little while ago. Why wouldn't that have made them hide? Did they start on this afterwards? Or did they panic at that point, and the noise was something completely different.

Well. The important thing is, there might be a desperate person hiding in a room with two children in it. And if that's the case, they might try to take hostages.

You don't think you can take that chance. You should at least try to figure out if there's anyone else in the room. You open the door and slip in. You look around for signs that either someone's here, or of what else might have made the noise.

And then you spot something. Something that you couldn't see from outside, but that's even stranger than the glowing symbol on the floor.