As much as Cas didn't want to leave Anna alone in the building, much less with people they didn't know, he nodded towards her. "I'll return shortly. When this is over...we'll deal with formal introductions."
He was curious about both men in regard to how they knew Mary - even Rob, considering there was so little he knew about the other man in general. Like all Winchesters, he was difficult for Cas to read. "Rob, if you would accompany me."
Moving down the hall as quickly as possible, he moved in the general sound of the gunshot. It wasn't especially easy to trace, but once one had tracked a single shot, then it become something like second nature. Hunters got shot, sometimes by friendly fire and other times by not so friendly fire. But each time he turned a corner, he made certain to check around it, pistol drawn each time. As for Rob, he trusted the other man to watch his back.
All he could hope was that the shot was not fatal.