Ball Python escape, could he have gotten out of the house?

[Bogertophis]

Okay it's morning, there was no trace of movement in the flour, the rat is still uneaten and there's still no sign of him.

Part of me is worried he wouldn't be able to take the cold, it got down to 16 degrees celsius in the night - when we checked behind the furniture fridge etc last night there was no trace of him at all, so I'm genuinely not sure he's even in the house

If your house is cold- remember that floors are colder than higher up- your snake will either be hiding near a source of warmth- something like the running motor UNDER your refrigerator- which can be VERY dangerous for a snake- or he'll be too chilly to move around much at all. Keep looking- many times loose snakes are not found for a few days or more- you've only just started. I get your frustration- most of us have been there, & once you have, you'll be much more motivated to never allow a snake to escape again.