Aw man, I really feel for you. I'm in the process of moving, and was gone for about a week. When I got home a bunch of my corn's bins were closed incorrectly, VERY close to the edge. One of the bins (where the most valuable snake in my collection resides of course) was even open just a slight crack.
Thankfully all of my snakes were in their rightful places. I can't even begin to describe how relieved I was. I think my remaining roommate must have been showing them off to someone at some point during the week and didn't push the bins back in all the way. I'm just so glad there were no escapes!
Perhaps try placing hides with heating pads around the basement so the snake will be attracted to the warmth. Once I had a baby escape during winter and crawled onto me. I woke up with her on my face. I must have been the warmest thing in the room, and I'm pretty thankful for it. That was a VERY close call.
Another past escapee years back was found hiding out under the hot tub.
Good luck, keep faith. All I can add is looking back, I'm pretty thankful for my rack! It's pretty much escape proof, thank goodness.