First, check all around his cage. Lift it and any surrounding cages up and see if the snake is hiding under the cage. Take everything out of the cage and sift through it, to see if he is hiding in the substrate, under the water bowl, tucked on the underside of the hides, etc.
Then, start searching EVERYWHERE! Look high (on top of pix frames, window blinds, etc), look low (under sofas, fridges, stoves (but don't squish him). Look inside laundry baskets, inside the clothes, etc. Look inside shoes. Look behind books. Look right next to and underneath his tank. Search very thoroughly boxes and baskets and bags. Look inside any cabinets and drawers. Don't forget the bathroom.
Some people will put flour on the floor right next to the walls (snakes can hugs the borders of rooms to travel) to see where their snake has been. Some people will sit up at night with a flashlight and turn it on every once in a while to check for the snake. If you have any animals, watch their behavior. If the cat is watching an area very carefully, go check to see if your snake is there.
Seal off rooms after you search them, as snakes have been known to return to a room that you have already searched. Make sure that no gaps occur in the towels that you stuff under doors, etc.
Then I would suggest making a snake trap:
It is pretty simple. Use an appropriate size plastic soda bottle, for a small snake use a 16-20 oz bottle, for a larger snake use a 2 litre bottle. Cut the top off the bottle right where it is the largest. Then insert the top of the bottle backwards into the bottom of the bottle. What you are doing is making a fancy funnel type trap. Put the trap that you have baited with a frozen/thawed mouse of the appropriate size in the house where you think they may be on the floor. You can put one of those heat pads under it. Leave it overnight. In the morning, hopefully you will come out to a sleeping snake in the bottle.
You might want to make a few of these to use in several rooms. Don't forget the hallway and bathrooms.
You can also post this question in the escaped forum on kingsnake.com for more help.
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sue