Check in around and under the rat cages. We found Kish under the My Pet homes. I had looked 2x, but found him coming out a day or two later.

Cool down the house, except for one room.

Can put flour along the doors and walls to capture his travel direction and to see if he moves from room to room.

Laundry piles/hampers/backpacks are good hiding places

How big is he?