How does Injun Joe figure out that someone has been in the house ? Why doesn’t he look upstairs?

Injun Joe notes the tools they've found, as well as the fact they've recently been used. He isn't worried about anyone being upstairs because he was ready for them.... and figured they'd run away.

“Now what’s the use of all that? If it’s anybody, and they’re up there, let them stay there—who cares? If they want to jump down, now, and get into trouble, who objects? It will be dark in fifteen minutes—and then let them follow us if they want to. I’m willing. In my opinion, whoever hove those things in here caught a sight of us and took us for ghosts or devils or something. I’ll bet they’re running yet.”