I think most everyone has had moments where they think there's some kind of UAV hack or something. What I find most often, is that I've been hit with a Flash or I killed someone who was on a long Deathstreak.
It does get annoying when you're seeing on Killcam people turning corners as if they know you're right there. I just chalk it up to the reasons above
Recon Pro users?
Recon Drone users?
Revenge Death Streak?
Probably got you on radar.
However, I have gotten extremely good at hearing silencers being shot, so it would raise my defenses. Also, bullet fire can be seen so they could have seen you shooting.
Also, spawning near the person that killed you. He would have an idea of where you were.