It is a horrible feeling...my bull snake got out less than a week ago...his screen had a broken latch in one corner, so I used a couple of 2and a half pound plates in that corner...found the plates on the floor...didn't think he had it in him. I heard a rustle in the closet and found him in a box of old junk, under other boxes and an old computer keyboard...probably wouldn't have dug that far into the box if I didn't hear him rustling against the cardboard. Try sitting quietly in some of the bedrooms with no tv or any other noise and just listen...especially after turning on or off the lights.
Good luck and keep us posted