There's no red herrings here; he hasn't left anything of value in the garage.
Remember, he was round last Friday claiming he had nowhere to live and was going to park up outside your home for the night?
Barging in and snatching what he thought was the key to garage was an attempt to enact a scene from one of St Twuntbert's ploys plays wherein, having gained access to the building, he'd set himself up with a sleeping bag and a primus stove in the hope that you'd believe his spiel about being homeless and the sight of him becoming increasingly unkempt would melt your heart.
In his dreams mind, frequent knocks on your door for water, use of the loo/shower, and other sundries, would give him the opportunity to wear you down until you told him that he might as well move back in.
The clue to this man is that he wants what he wants when he wants it and he doesn't see any reason why he should have to wait for what he wants.
You've indulged him far too much, Starting. He's become accustomed to getting his own way with you and this has done nothing to address the fact that he's always been a spoilt, selfish, petulant individual.
The only reason you're surprised he's behaved as he has is that, as you haven't crossed him before, he's succeeded in hiding his true character from you.
But I'm guessing that if you think back, you'll see the signs that you chose to ignore at the time.