I'm guessing they ain't coming back,
In the unlikely event they did, I'd say the night security camera spotted someone dropping a gun off the other night, and the police picked up the tape and the gun and are keeping it in safe hands for the owner to go down to the station and pick it up.
Also, tell them you get guns, but are more an AK guy, and that AR build was such an embarrassing POS you'd drop it off in someone else's yard too, like a fat girl and a moped- too ashamed to be seen with it. "Now... get off my lawn...(ahem... new cement patio) you dirty rat... 