Keep looking! Look in every nook and cranny you can think of, especially warm spots. Behind/under the fridge, near the back of a computer, anything. I made the same mistake as you when I purchased a kingsnake 2 years ago, I put the lid on the ten gallon the pet-store said would be safe, but he found a way out. Try and keep the apartment cool, and keep the tank open - with lots of warmth. You can even plug in a human heating pad and set it on the floor in the middle of the room. I know there are other tricks, but I can't think of any so lets see if others can help you too.
I found my kingsnake 6 months later in the closet, so don't give up. Unfortunately I believe someone let him out when I was not home, but he did disappear a few weeks after I found him
This is what i am using for my new albino kingsnake - when he is fatter he will go in a ten gallon tank but until then, this thing is escape proof! lol The clips are quite strong.
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