That's sad keep looking/doing the flour thing. He can last a long time out there if he finds the right spot and they can get into places you wouldn't dream of.
Sounds like you're going the right way about looking for him, which is all you can do at this point. No point beating yourself up over it now, chalk it up as a lesson learned about tank security for when he shows himself again!

The flour trick is sound- I've never had a vanished snake but have used it on other escaped critters and got them back. Hope you find him soon, fingers crossed for you. :3