Not theft, but still conversion. The thief could sue you for the value of the items. When a car is repossessed the chattels (personal items) have to be returned to avoid liability. There is case law establishing that if you tell the repo agent you have stuff in the trunk and he drives off anyway, that's prima facie conversion (monetary liability) regardless of whether they try to return the items later.
There is one wrinkle in the intent issue, which is that you may not have had intent to take the item when you drove off, but if you later discover it and didn't give it back, you might form the requisite intent at that time. All in all, this sounds like a good law school exam question!
1: https://california-business-lawyer-corporate-lawyer.com/cali...
It seems extremely unlikely that the simple repossession of a car with personal property is conversion given that roughly 100% of all repos will have at least some personal property within.
It’s clear they are obligated to return it to you, but your argument here doesn’t sound right.
Even more to the point, driving your own car is not stealing it, either.
Personally, if found stuff that wasn't mine, it'd get pitched to a dumpster.
Or even much worse, and you get blamed for the item and go to prison.
Once you found the backpack, you would have to return it to the thief for it not to be theft.
Hopefully you wouldn't get prosecuted, but you are then at the whim of the prosecutor. If the thief calls it in you've created a bad situation. I was in jail and saw weird crimes like this. A guy bought some dope for $40 from a dealer, but only handed him $20 and ran off with the dope. The dealer called 911 and reported the stolen merch. The buyer was in jail with me. WTF.
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.