Disappearing George

A couple of weeks ago I posted about our new cat, George. Well, he's becoming a bit of a handful.  One day last week we lost him. He wasn't around when we got up, didn't come to be fed all day, didn't use his litter tray. Three of us searched the house from top to bottom, twice. Looked under every bed, in cupboards, behind sofas, nothing. So, we decided he must have got out somehow. Stu and Luke went into the garden and searched that, but nothing. A few minutes later, there he was on the kitchen stairs, just wanting to be fed. So, he'd been inside all the time. No idea where. Yesterday morning we lost him again, this time he'd got out of the cat flap, which I'd unlocked. So, we searched the garden, our neighbours' gardens, the surrounding streets, nothing. He still hadn't appeared by midnight, and we knew he hadn't eaten so we were worried. At 1am I went searching again, nothing. Woke at 5.30 so tried again. This time - success - he was in our neighbour's garden, and came when I called him.

Picked him, up, gave him a cuddle, brought him inside fed him, and locked the cat flap. That last bit is important!  Went back to bed. Got up a few hours later - and he's gone again. This time we know he's inside. But where?? It's now 6pm and we haven't seen him for over 12 hours - he hasn't appeared, made a noise, or eaten. Where the hell is he??? He's HUGE, and we live in a small house - where on earth can a cat of this size hide, and why?

Edit:

So there we were eating our Christmas Eve meal, when we saw him in the garden, and he calmly came through the cat flap (which was only locked one way, which we now know was important....)
he ate, was fussed, and tried to get out again. Which was when we realised he was getting though the locked cat flap by getting his paw under it and pulling it towards him. Clever cat!