I'm so frustrated. My 3yo has developed a habit of taking things, important things like debit cards, keys and phones/cameras etc, and hiding them. He'll do it very deliberately, the object could be on the kitchen table where we'd just put it a second before and when we turn around it's gone and so is he. We'll ask him if he's seen the item and he'll tell us that he hid it, but he'll refuse to tell us where. He hides these things so well. We had the house upside down once looking for the camera, which he'd hidden in the bag of potatoes in the pantry. Other things are similarly ingeniously hidden 
We've tried time-outs, taking things away until the item's returned, taking away treats and nice things and out of sheer desperation, sometimes even trying to bribe him to tell us where the things are (like when he hid the house keys - they were in his tub of army men, buried in his toy box) but he won't even take the bribe. He plays dumb as soon as he's admitted to taking it, "I don't know" "I forgot" or just plain "I'm not telling you".
I don't want to resort to having to hide things away from him, but I also don't know why he does it and if I can't solve this problem he has, I think to save my sanity I'll have to. It's SO ANNOYING.
Tell me it's a phase that children go through and he won't grow up to be a kleptomaniac, please...