Hmmm not convinced
I guess a counter argument to that with would be a stealthy bow Ranger trying to take out some sentries quickly and quietly with the benefit of Hunter's Quarry. Would be silly if a huge WoW-like pointy arrow appeared above the sentry's head?!
Reebo
Uhm....the rules explicitly say any target is aware of any conditions affecting it and who caused the condition. There's not much room for "convincing" there. You can houserule if you wish to, of course, but the rules as written are crystal clear.
Now, it does not follow that OTHERS know what's going on -- that depends on the "special effects". But since the person targeted DOES know, he is perfectly free to communicate this information:"Guys, I'm feeling all woozy and weak...it's coming from over there, in the woods!", or whatever.
Remember, always, that the NPCs live in the world and that (barring unusual circumstances) the powers and abilities PCs possess are not new or unique. Kobolds, orcs, etc, can have their own warlocks and rangers, and even if they don't, they've dealt with them enough times that they know what they can do and will have developed strategies and tactics to respond to it. It's like saying that if a modern soldier doesn't happen to have a grenade of his own, he won't know what it means when he sees one land next to him. He knows what it is, what it can do, and can quickly deduce that, duh, someone must have thrown it and probably guess, roughly, where it came from.
It should also be noted that the rules say "When you affect a creature with a power" and that creatures know what "conditions" you have imposed. Hunter's Quarry is NOT a power and does not impose a condition, so it might be an exception.