We all know Nina is a powerful woman, with lots of connections. She could be having Peter followed and watched, too. Her henchmen have the resources of MD at their command, so maybe they're surreptitiously picking off all the hit men who come for Peter. I sorta like this idea, because it ties in with lots of forgotten story lines. Nina knows Peter still owes her a favor - and she *does* care about him - so he's valuable to her and she would expend the effort to keep him around.