If its inside his car, then nobody should "find" it, because nobody should be going through or opening his car.
As far as things leaving the premisis goes, if you take $20 off of my dresser, without my permission, and you put it in your pocket, you stole it. Even if you have not left my property, you stole the money. Even if you put it back, or put it in my car, you still stole it.
Now think about this, the person that went into his car was also TRESPASSING!!!!!!! Thats a crime punishable by death most of the time around here, isnt it?