It's very bizarre that he would steal a pink hat.
Why do you think it's bizarre?
Because it's not an item of any value really. (Not that its having value would make it any more or less OK - it didn't belong to him, period). And a little icky to take a used hat to give to his wife. (If, indeed, this is what happened). I would think it extremely bizarre if I couldn't leave a simple item like a hat or a scarf on my desk in the area designated for me in my workplace, where I know and greet the same people every day and nobody else has access, and find it still there the next day.
I would find it less bizarre if money went missing. I'd be outraged to know that a thief was in our midst, but not puzzled.
Someone once stole a $5 pair of scissors from my kitchen drawer. I knew who it was because of her history, her character, and the fact that she'd been the only one who had visited the house. Those scissors never did turn up even after a move. I thought it was a very bizarre thing to steal, when there were other items around of higher value. But then I googled kleptomania. In the OP's case though, it sounds as though this wasn't just a random compulsion to steal worthless items, but a planned theft and I find it really weird that he wanted a specific USED pink hat for his wife so badly that he'd steal it.