Sometimes when I’m bored I like to REALLY push the feeling; I try as hard as I can to get something to happen, like an entity jumpscaring me, by acting clueless.

If I had to guess, it might be some combination of lighting and ambient sound. Source engine lighting may fit into the uncanny valley. It's not so bad that you immediately understand it as stylized/artistic, or 1990s, but it's not super-realistic, either. Memories about source engine games are comparable to real-life memories, as the lighting quality is detailed enough to be somewhat realistic, but the human brain isn't capable of visualizing real-life memories beyond these early 2000s heuristics. The uncomfortable impression that late-night Garry's mod sessions leaves upon the soul might be because the memories you develop seem almost real, but a bit off. The ambient sounds do nothing to comfort the player, either.

Personally, I'd say there are three main reasons Source games can feel so scary: 1. In some cases, the game IS a horror game, so it's supposed to feel scary. Like for Half-Life, Left 4 Dead and Portal, those series are kind of horror games, so it's unsurprising to me that they would be unnerving: that's the point. 2. Emptiness of the map. Most maps in these games can feel empty when played alone or with no NPCs, and the feeling of loneliness in a big map will quickly lead to the delusion that you aren't actually alone. A game called "0 Players Online" was made a few years back about this exact ideology, it's literally a fake 1990s online FPS game with one big map and no other players in the session, yet you start to see demonic ghostly figures stalking you (which is explained to not be a delusion but whatever, close enough.) 3. Sound design and lighting. The fact most times, there is no music in the background, it again invokes that feeling of emptiness and loneliness, music has a major emotional impact on the consumer when used properly, that's why in many horror movies or horror games, the music will cut out before a scare. In these Source games, the music is automatically non-existent, so it's like your constantly on edge for a jumpscare that never comes. The lighting can also have drastic effect on the user, setting the tone of a scene. If it's dark and there isn't much light anywhere around, you'll get an uneasy feeling because you don't quite know what is around you anymore, since you vision is obscured. Lots of these games use lighting strategically to mess with the player.

Back in the days, after discovering 'noclip' and messing around with Worldcraft/Hammer, I realized why Source games are eerie and unsettling - all you're allowed to see is a well-crafted coulisse with scripted sequences and most of the ambience sound does not originate from anywhere.

I think its the emptiness mostly. Garry's mod in particular is incredibly empty, things don't move particularly much. We are used to having a lot of micro movements in games nowadays. Wind affecting the leaves, grass, bushes. Birds flying by. Clouds moving in the sky. People and cars passing by. Particles like smoke coming from vents and so on. Things feel unnerving because there is no sign of life. It probably makes your brain draw conclusions that life isn't being actively supported and therefore it puts you on edge. I believe emptiness itself isn't necessarily scary. In a certain context, however, it is. If you stand on a well lit flat plane where you can clearly see everything within miles then it isn't so creepy. You expect emptiness. But in the context of being in a place where you expect some movement, it suddenly puts you on edge. Standing in the yard of a school. Not a single person in sight, theres a flag but it's still. The leaves aren't moving. There's no wind. No movement whatsoever. The only moving thing is you. That feeling of wrongness is probably the cause. The lighting of course helps with the creepiness factor. In source games if the area is well lit, it isn't so creepy. That applies for most things though. We find comfort in being able to see things. When we can't it triggers that fear of the unknown.

You are wrong, the source engine is mildly realist, the thing is: you hear the ambience and you see nobody, you feel like you are stuck in a paralel dimension listening to the real one where everyone else is. Gold source was also scary, look at some cs1.6 maps and tell me if you don't feel the same. Valve is making horror involuntary, i wonder if they make it on purpose what will it be like.

unfortunately, this point does not hold up then you are creating creatures, building helicopters, or killing your friends for sport.

We get a video per year about this topic, lol.

Imagine being a schizo in a vr mod of gmod in gm_construct

ooooo old technology creepy...smh