The exact same thing happened to me last year in July - August. I went to go feed my leo Anubis but he wasnt there. I looked all over the house tried putting flour down so that if he walked trough it I could follow the food prints. I also tried putting crickets in a cage in the my room, I was hoping that the sound of crickets chirping would attract him but I had no luck. Weeks went by and I had pretty much given up. I thought I would never see him again. Then this february my mom was in the bathroom brushing her teeth when she saw my gecko beside our bathroom cabinet. I had just got off ssnakess.com chat when she called me. I ran to the bathroom and sure enough there was Anubis. I couldn't believe he was still alive he had been missing for 6-7 months!deh-
yrated and extremely skinny. I gave hime some liquid calcium, water, other minerals and a few crickets. Although he didn't eat the crickets he drank for at least half an hour. The next day he ate and has been eating like a pig ever since and is now getting nice and fat!
My advice to you is try some of the above techniques including putting down flower, and linds' idea awell. Keep looking, look in warm spots like behind the T.V near the vent,andaround other you other reptiles heat sources. Good luck!
Here is a pic I took of Anubis yesterday March 30 2003