Well first of all, don't give up!
How far was his tank from the fresh outside air (open doors)? In my experience, snakes ARE drawn to fresh outside air, but BPs usually lay low during daylight hours & move around during the "cover" of darkness. They also don't often travel far, so LOOK in all the nooks & crannies: under beds, & under & in couch cushions, gaps under kitchen & bathroom cupboards, in laundry baskets full of clothes and empty out your closets, because they often end up in there. Upholstered furniture including beds often have tears in the fabric on their underside, or stitching gaps in the crevices under cushions- so look before you sit. Stay up late in semi-darkness & LISTEN for him too.
Snakes are far better at "hide & seek" than we are- most of us have found out the hard way& most of us find our snakes unharmed, not counting our stress & sleepless nights. Hang in there.
Good luck!