I think one of the common suggestions is to put flour/powdered sugar/sugar over doorways or in other areas so you'll be able to see if she's moving around in case you don't hear the plastic crinkling. Have you tried checking the vents? If she went in them and you closed them off, it's possible she's trapped inside them now. I would make sure your cats are confined to rooms you're sure your snake's not hiding in so they don't find her while she's loose and then kill her.