If your house has a room where two cabinets meet a right angles, forming an inside corner, there is a space he may get into, depending on his size. Feel under the toe-kick space where the cabinets meet. There is usually a vacant place an ig can easily find.
Have you checked the closets? I've found my igs clinging to the fabric of hanging clothes. It's very natural for them to hang vertically from their toenails. They can also cling to curtains and fireplace masonry.
don't step on anything you can pick up. he might be hiding under a t-shirt.
Check all the blinds, curtains, and valences three times.
This might sound implausible, but if he's laying on a shelf of some kind, you can look right at him and miss it. That's happened to me.
Do you have a yard, or an apartment? It's' easy for igs to get outside. They normally hug the foundation as they creep along looking for a hiding place. Once one of my big igs hid under a lacework of blackberry vines, fully visible, but the vines tricked my eyes. A helper finally found him by tapping along the edge of the house with a three foot long stick.
Igs can also travel to adjacent streets, and be found by strangers. Some are terrified by them, and others know how to walk right up and pick them up. So make 30 or so fliers and post them around your house, and along the houses behind your back yard.
Are there trees he can easily reach? Study each of them using binoculars, looking for your ig's head protruding as he looks down from a branch he is laying on.
That's all for now. When I think of some more, I'll post them.