@maxterdexter:
Can observation haki tell you if it's an specific person? Or you need to feel them and identify them before remembering their "voice".
My guess is that the radar part of observation is a lot more complicated than the "attack detection" part, and need to be activated conciously, so it's not like "oh Absalom is walking around" and more like "If I turned on my observation power, I'd feel a guy nearby, eh it's probably a mook from their crews listening in".
tldr;Theory based on observation of the observation haki: The attack detection is a sense that it's easy to keep on, while the hearing of voices is more complicated and needed to focus (Except in freak cases, Like Aisa or Cobby)
I think you're on the right track. That was how it worked on Dragon Ball, which is one of Oda's biggest inspirations, though it doesn't make it credible fact.
I'll cite instances from Fishman Island and Punk Hazard. In Fishman Island, Luffy, Zoro, and Sanji all felt Caribou in the palace. They didn't recognize his "haki signiture" I'll call it, but then again, they had only met the guy once. They weren't looking for him at the time, but after kicking him out of the castle, had to hunt him down in the mainland portion of the Island.
In Punk Hazard, Sanji dove into the sea-lake to find Kinemon's torso. He's can't find it right away, saying he can't tell apart his "signiature" from the sharks'. Of course, that was because the torso was getting nommed on by said shark.
So yeah, like I said, your thoughts are headed in the right direction and just needed a little evidence, a little more meat to back it up. Which I tried to cook up for ya.