Absolutely on Team Turn It In. Yesterday, my kids found a plastic grenade at the park (probably valued at 50 cents if that!) and my 2yo went to pick it up and I was all "Oh no, we have to leave it where we found it in case the little boy who lost it comes back to try and find it. Think how sad you would be if you left a toy somewhere on accident and then couldn't find it." (ok, I was scared it was dirty too lol).
But I won't judge the cousin too harshly. I was checking out at the store today and my kids literally smacked each other 5 times before I registered what was going on and put a stop to it. Sometimes you just get in the zone (or at least I do) and you don't think about things your kids are saying lol.