I hate to say this, But I need to be honest about the possibilities.
1. They are excellent climbers, so if it got out on the roof it's no problem for that lizard to leave.
2. they command territories that can cover miles in the wild, and once motivated can go great distances swiftly.
3. they can squeeze into really tight spots, I saw my fully grown Savannah Monitor squeeze through a gap less than two inches wide before. (Like a rat, if they can get their head through the hole, the rest will follow)
4. If it did get outside, the 40 degree nights have most likely taken the toll.