So i'm sneaking around a spot and its filled with gunners. I'm fine and behind something. A ghoul attacks me so i killed it with melee but i'm still in danger. Turns out somehow the gunners off in the distance where alerted to my location cause they somehow saw me cause the ghoul could see me. So when one thing sees you they all see you. Even when you're behind stuff. So i get things lobbed at me like rain of grenades since enemies have infinite grenades but once the ghoul died they all stopped.

It may be my imagination, but Gunners seem to be the most perceptive human mobs out there - they can see (and hear) surprisingly far at times. My guess, though, is that they were responding to the ghoul, not you, since they lost interest once the ghoul was dead. Ghouls are noisy. Of course, how they knew the ghoul died is another matter...

Also, mobs of all kinds will throw any old area-of-effect attack in the general direction of the last known location of an active, but hidden, threat. I've had Super Mutants attempt to blanket my sniping positions with missiles, for example, after I'd snuck well away from that position, and I couldn't count the number of times raiders have lobbed grenades in my general direction after I'd found cover. The general rule seems to be that If they see you, they'll come for you; if not, they'll throw a volley of spray-and-pray fire at you, then wait for you to show your face again before firing.

It is uncanny that mobs can telegraph focus-fire positions to each other so precisely, though.

It's one of those old carry-over things that show why Bethesda should update their engine.

It could be that they heard you kill the ghoul. Sound impacts stealth as well. But I did not witness it so thats my best guess

That's probably it. In Fallout 3 and New Vegas mobs magically knew if you'd attacked someone. It was especially annoying as a stealth build, because I'd get off a perfect stealth kill with a silenced weapon and somehow every single person in the cell immediately knew not only that one of their own was dead, but that it was me who did it.

I dunno, they also magically forget when the guy standing next to him head explodes. They forget they got wounded a minuet ago....

Yeah... mobs/enemies will shoot in your general direction if they get paranoid enough. You can however use this to your advantage if you are sneaky enough and outdoors. Make a lot of noise in one location and then move to another and the enemies will keep on attacking and searching the first location. This means you can plant mines...get enemies into a better position for sniping and all sorts of things.

This tactic doesn't work at all to your advantage inside. If you make noise, enemies will usually stick to covered hiding spaces and won't leave a room at all. . They can also spot mines and will shoot at them to detonate them sometimes.

It's all that jet, makes you crazy. "Huh. She was right."

My favourite example of this is Skyrim's "Must've been the wind." when the guy has 5 Arrows sticking out of his face.

That's one of those "classic" Bethesda RPG tropes.