Chalk one up to minor miracles - in spite of all obstacles, they'd found their way to a hiding place. John honestly hadn't been prepared for success, so he couldn't help but smile as he barred the door with a cleaning cart. It was mad, running and hiding from the nurses, but they were in a madhouse. When in Rome.
As the sound of his name, Watson turned around. Why was she asking about his -- oh. Oh. "You were listening. I'm sorry, that probably made for fairly grisly entertainment. Yes, Sherlock. He's my flatmate. A 'consulting detective,' and before you say it, yes. I know that's not a real occupation. He does it anyway."
John kept his voice down - just barely over a whisper, as he was trying to listen for signs of commotion in the hallway.