Put the flour on doorways, set up heatpad /mouse/waterbowl in all rooms upstairs, no sign of activity. Going to under the floor down stairs and set up heatpad/ mouse, see if he's down there. If he takes a mouse least i know he's around.

Will not lose hope as they can turn up months later....by the way ..did I mention the snake is not mine, I'm looking after it(not very well) for a few months!