That IS the point! He knew it was missing because he knew where it should have been! Quite simple really.
But Caroline, first you don't know that he knew where it normally was. You're assuming that. Just because he's looking for the wallet and he notices that it's missing, it doesn't mean he knows exactly where it should be. He could easily intuit where it should be, just by using common sense: just look in the master bedroom, where Nevill slept, and where he would naturally keep the wallet; if not there, then look in the office. If he doesn't find it in the expected places, then it's 'missing'. As I have just explained, his father was found bare foot in his pyjamas, so the wallet must be somewhere in the house.
And even if he did know where the wallet should be, so what? That could well be because he just knew for innocent reasons. It's his dad. They work together. He knows the house. He will have recalled where his father kept the wallet in the past. So? It can be innocently explained and it doesn't follow he was at the scene when the shootings took place, but in any event, you can't prove he knew in the first place.
Can I add that you haven't shown us the letters, or you've taken them down. Having looked through some of the thread, I'm not clear why you did that. Without sight of the letters, it's still unclear to me what you're driving at. So far, I see nothing suspicious. It's his father and he's the only surviving next-of-kin. He's looking for his wallet. That's exactly what I would expect him to do. It's what anybody would do in the same situation.
