Corns are notorious escape artists. Don't be so hard on yourself. Do check vents and any other potential exit points, however high up the wall. Closing them off at this point may lock your snake in the vent system if he's in there, but could also prevent him from getting in if he hasn't already figured that out. Be wary of possible spaces under doors (if they can muscle their heads through, the rest will follow), and keep the rooms you've targeted dark, which will encourage your snake to venture into the room more than a light space will. Hopefully a dark, quiet room will yield some clues, if not immediately return your snake to you. It may take time to find him...be patient and wait it out. Good luck, and here's hoping you find him safe and sound quickly!