My inkling, as a former lawyer, is that this would not be theft if you didn't know the item was there. Crimes require a certain mental state, and theft typically requires that you be aware that you're taking a thing that doesn't belong to you. If you leave a restaurant and unknowingly take someone else's umbrella from the bin instead of your own, you cannot be found guilty of theft.
If you do know the items are there, you could be found guilty. Of course, the practical reality is that you don't want to give a criminal any reason to come back to your house, hunting for his belongings.