I recently ran into a very funny little issue with the latest patch. I doubt I could tell you the exact steps I made inside the game, because I was quite a bit into it. I entered the NCR Bazaar area and left Marcus there. I did the map quest for the slavers, after which I wiped them out for the rangers. I killed Lenny for Merk in order to gain his respect, after which he gave me the quest to bring the doctor's papers to him. I did this, and afterwards proceeded to kill Merk. I also needed to kill his 2 bodyguards and Mira because of this. After this, I went all around the main map (Vault 13, San Fran, Military Base, whatever), and the next time I came back to the Bazaar, the entire map was reset (but only the Bazaar, thankfully) and Marcus vanished! Mira and Merk's 2 bodyguards reappeared, as well as the slavers, but not Merk himself. I'm no programmer, but if I'd have to guess, I think this little bug probably has to do with Merk and his bodyguards shifting places and being inside the city at day and in the bar at night.