Perhaps Jarvis sensed that Master Jamie was on some sort of ledge. Or perhaps he just saw the weariness that she tried to hide, and probably succeeded in doing so from nearly everyone. Or perhaps he was just annoyed at Phone's attitude. 

He rose, an intimidating height and build, his expression would probably be enough to stop near anyone in their tracks right then. "It's not a tattoo, it's a Mark. And obviously you need some watching, as you go around angering guild magicians and destroying train carts. What you are going to do now is sit there, and shut up"