Kingsnakes are masters of escape. Goodluck in searching.

Best thing is to make sure it can not get out of the room. Snakes usually do not travel far from their enclosure but you want to make your search area as small as possible.

I would check the dark places, higher places, and tight places. Snakes will usually travel along the wall before coming out into open space so follow the wall

Turn the lights off in the room, check on the room every 15 minutes or so, and see if you can 'bust it' moving across the floor.

Those only work if you luck out though Best bet is to sort through everything in the room.

If all else fails, painter's tape. This can be dangerous for the snake but it has worked for me in the past. Strech out a couple lengths of thin painter's tape across the floor...sticky side up. Make sure both ends are held firmly or you will end up with a mess. Then wait it out and check the room every hour or so. Even painter's tape can be a pain to remove from a snake, so make sure you pull 'with the scales' not 'against the scales'.....and keep a container of olive oil(or similar) around to put on the tape once it gets peeled off....or it will end up stuck twice. REMEMBER....this can be dangerous for the snake....so it is only a last resort.