Was the Barred Rock one of the top hens? I'm sorry for your loss and that you now have to deal with getting rid of a rooster you didn't want and was dumped on you.
Likely some A-hole tossed him in there during the night.
It truly is a sad state of affairs when we have to protect our chickens from human predators.
I'd get a lock on the door.
I'm wondering if some opened the gate to let him in thinking he was mine.
You would think they would come and ask before letting him in if that was what happened.