Mark Whalberg's character, Dingam, was cosigner to the apartment to which Sullivan was uncomfortable to admit. You figure this out by the end of the movie as to how he was able to be inside the apartment to kill Sullivan unless he had a key. Sullivan wasn't even surprised that Dingam was there, he gave the look like it was to be expected. Plus there were signs of each other's sexual relationship tensions between the two at the workplace. And Dingam frustration which had him to decided to move out. This explains the part as to WHY Dingam is nowhere to be found according to Sullivan... how is that possible if all you have to do is look up his address and go to where he lives, UNLESS that address was his place and he moved out.