So many layers.

1. What enemies can and cannot see is not always reported correctly by red zones. Apply logic to their cone of vision.
2. Sometimes enemies can incorrectly see through walls (and windows), especially in Act 3. They can see you on the xth floor from the ground up as well. This is why you should always be in stealth mode when doing something illegal. It makes preternatural instances far more rare. No stealth check forced on them = enemy automatically detects your action = bad.
3. The person you've just killed can report you to allies one second before death in some circumstances. Condition one: they're made temporarily hostile by attack, condition two: the damage takes long enough to calculate (so especially multiple sources/stacks of damage, etc.). Can only force a stealth check to try to avoid it.
4. Some goblins are indeed all knowing, have been since early access, but that's a goblin camp thing. This is usually a problem with theft for me, and a certain item that's not even marked in red, but I'll absolutely believe someone may get this with other crimes.
5. This is pure speculation based on Skyrim, but the "person" witnessing the crimes could be something that shouldn't be able to -- a rat, a rock, etc. This area is uniquely problematic in that you'll never know for *sure* if you're unseen, even with no lifeform around within 50 feet.

Against what others have said, in my experience, goblins completely ignore blood. It shouldn't be causing any reaction. The level 3 guard you're talking about is an absolute dumbass as well. If everything works, it's remarkably easy to wipe out portions of the stronghold at a time. You only have to commit once you attack Minthara or Ragzlin. However, Ragzlin can sense bear Halsin from remarkably far away: this is because there's an actual hole in that wall.

The moment you unstealth (remember, red zones CANNOT be trusted, they're a guide!), it may be chaos. A red zone specifically means that, in this zone, an enemy will FORCE A STEALTH CHECK. it does not mean that this is their entire area of perception. Those two usually overlap and are somewhat similar, but almost never identical. It's also why you have to stealth outside the red zone, even when no one's testing your stealth. Somewhat annoyingly, enemies can force a stealth check outside the red zone in combat, but not usually outside of combat. Really, you're spoiled in terms of causes of odd behaviour in the goblin camp.