I guess I would start a methodical search starting by looking behind around and under the tank area. (You probably already did) then start looking in dark places like under chairs, beds, dressers, even under the fridge. Regardless of where you might think they can fit, because they have a way of getting into areas you think are too small for them. (Kind of like a mouse) Many reptiles, etc. can fit in places 1/2 the size of their actual size...some even smaller. Then I would take my time and look in, on, and under everything by dividing the room in sections. One way I help keep on track and focus on small areas at a time is to use a flashlight (even in the day sometimes) and just keep following the light.
Good luck I hope you find "Houdini"