You're going to think I'm nuts for suggesting what I'm about to but try to stay w/me. This is an old, really old, method for finding lost objects. Ok, Max isn't an object per se but it still applies. Turn a glass upside down and ask "Where is Max?". Keep the glass upside down until you get your answer. It may not bring him back but it could suddenly occur to you where he is so that you can find him. This method originated in Romania (I think) and, admittedly, it's odd but it does work. It stirs your subconscious so that you'll see results. Sometimes it can take up to a week or more to work but I've seen results in a much shorter period of time. Even if you decide to think I'm batty (that's ok; I can take it), at least don't lose hope. He was happy at your house and he's not stupid. Animals go where the love is.