He was in the basement after all! Saige and I had both searched down there, and called him, to no avail. I figured that he probably found some corner to die in. However, I just went to get Saige some ice water, and I heard Teo yowling. I figured out he was in the basement, and I found him, hiding behind part of an old cage. He fought me, a bit, as I carried him upstairs, but we made it. (It hurts him to be carried, but he was desperate to get upstairs, where he belongs!)
I am so glad. He might die, still, but hopefully, I can keep him in the room from now on.