Sorry this is kind of gonna be all over the place but
A lot of the HL2 (campaign based) mods I've played and finished are inexplicably interested in ghosts or otherwise haunted spaces. Which is interesting for a game like HL2 that really doesn't fuck with that type of thing.
And it's not really a super overt thing, it's usually just an Easter egg but like. The frequency that I see it combined with how similarly it's presented is. Interesting to say the least. Like the most Ghost acknowledgement we get in HL2 is a sound effect that plays by the apartment blocks and I'm inclined to believe it's less of "we are hearing dead children," and more of a "hey, you should be thinking: where are the children?" Because it's a relevant plot point that there are no more children. Not that they were wiped out just that we stopped having them.
Okay with that out of the way. Ghosts in HL mods. Starting with Entropy:Zero. Obviously spoilers ahead for literally every mod I talk about.
So in Entropy:Zero after raiding a rebel camp, you can encounter a radio that, when interacted with, plays the voice of a young girl that prompts you to find her doll. This is something you can do for an achievement in the game.
As an honorable mention- he's not a ghost but the cannibal in the next chapter is presented in a way that very much leads you to believe he is until you actually meet him.
Another honorable mention: whatever the fuck is going on with Wilson.
Okay next isss Entropy Zero Uprising. EZU has two instances of "ghosts" I wanna talk about. Neither of which are actually ghosts but like. Functionally they're ghosts ok. First and most obvious is Victor's wife. She's more of a dream/hallucination but she is very much haunting the narrative, calling out to Victor, and her appearance as a fuzzy, glitchy shadow is a kind of common way these ghosts are presented in HL2 mods.
There's also. The man. Idk how to really explain this one. He's a guy who shows up sometimes in EZU and he doesn't reeeeally do anything it's likely you never noticed him. But he is some Entity and he can kill you. Is he a ghost? Maybe. For the sake of this post yes.
Aaand finally because I'm getting tired but this is not an exhaustive list ok?
Dark Interval has several instances of ghosts and weird haunting type shit. In the apartments there's a guy in the dark who disappears once you look at him, he's presented similarly to Victor's Wife in EZU. I can't remember where exactly but there's a similar effect later in the game. Some shit falls off a shelf out of nowhere when you do a task in the early game. Dark Interval just has weird shit going on in general and I love it for that. ALSO THE PATH TO ELIS????