I think the killer would have simply followed Fahim to his apartment, and making it appear like he was waiting there. But, if he was there waiting, he could have been waiting at the complex indefinitely if the place didn't have a lot of traffic and there weren't a lot of people entering/exiting their residences. I would think the complex would have a pretty good CCTV coverage, but who knows? Let's be hopeful that the cops are looking into all these. If he was seen on that day alive and outside of his apartment, then the theory of the killer having spent substantial time in the apartment cannot be true.