We can try hitting them, to hear for the hollow sound.

[ This would probably be easier if he still had Kon with him, and Aki feels a little bit of an ache. The other Devil Hunters still have their contracts with him, but Aki feels a little bereft without his. They'd worked together for a while, after all, and even if it was Aki's fault that the contract had been destroyed... Well, he doesn't know if he can be blamed for feeling a little depressed about it.

Walking down the halls, Aki frowns. Where do they even begin...? He could ask Tsurumaru, but he sort of wants this to be some kind of surprise. ]

I'll check the left-hand walls, and you can do the right hand. If we trail our fingers and knock, we might hear something useful.