Im really not sure what more can be said to help you out. Did you look the hardest right around the area of the tub he was in? You say you put down flour but you have seen no tracks right? It sounds like he went somewhere and hasnt moved much if at all. When you look again, get on your hands and knees and crawl around. From that view look for anything he may have been able to climb up on. Anything he may have gotten under. I hope you find him. Its really stressful to have one of your snakes escape but you need to slow down and be calm when looking or you will miss something potentially important.