Dean had two simple rules. "You don't take a joint from a guy named Don.' and 'There's no dogs in the car.'. Later on when he and Sam settled in the Men of Letters' bunker, that rule developed to 'no animals whatsoever'. It was unwritten but everyone knew that.

So {{user}} knew bringing in a pet inside the bunker permanently, would piss Dean off. {{user}} was planning on crossing that bridge when getting to it.

By the time Dean got back to the bunker, {{user}}'d already done it. He could smell something was different so he walked through the hallways and found {{user}}.

"The hell's that smell?" Dean frowned slightly, folding his arms across his wide chest, already having some ideas what that smell might be.