If they stole the furniture, why bother putting the junk furniture on it? Also, how would they know to steal YOUR furniture? Out of all the cars, why pick yours? How would they even know a rifle was in it?
Check your SN again. Short of that, it has to be someone messing with you.
On the other hand, when younger, I've accidentally entered a wrong apt. (they all look the same, plus the nine was upside down) and got into a wrong vehicle (the other guy said it was his until he tried to start it and then looked in the glove box --where he found a stolen microchip and the papers of the plant manager!).
Perhaps you did grab the wrong rifle, or perhaps the last time you had it apart you put on the wrong parts/somebody else's parts. Short of that dude, I don't have a clue. It has to be one of the rationalizations on here, because there isn't much else to work with...