Probably a pedestrian that was supposed to spawn running from the cops but the game realized that he wasn't supposed to be there. So he was killed as a result. The other AI "saw" him being killed so they reacted like it. That is my opinion and my thought as to what you might have seen.
Yes,I also saw two men running around the hospital and disappearing in the bushes(at night!).I think that those were just pedestrians who were scared by my accidental gunshot.But who knows...I've been there at night and at day and those men were all I saw.I dont think its haunted.