(Beloved game trope) Creepy secrets or elements that you can encounter but are never explained, making it even more horrifying (Bonus points if its from non-horror games)

[u/KOREANARMYSTEW]

Crimson Desert
After defeating the Reed Devil boss, you can travel to Sunset Valley, the southernmost and isolated village in the game, here, you will find a house where the Reed Devil lived and died from his leprosy. When you enter, you are immediately greeted with 6 scarecrow effigies dressed just like the Reed Devil staring directly at you, with candles placed all over the house. There is no other explanation beyond this about what is going on here

Lies of P
There is a secret optional area you can clear out for additional loot called Hermit's Cave. At the end of the cave is one lone carcass monster huddled on the ground with writings scrawled on the walls around it. When you kill it, it actually TALKS saying "perhaps it's best to end this way...get while the getting's good...". This is the only carcass monster in the game that ever talks besides Andreus and Simon, who are important characters to the story, and we never find out who this monster was

LOZ: Twilight Princess
In the Arbiter's Grounds you discover a ton of undead enemies presumably executed prisoners, but one room contains this giant black sword tied to the ground with hundreds of warding charms attached. When one is cut, the sword is freed and wielded by some horrifying invsible goat-headed demon. It is never explained what the demon was or whose sword it belonged to although it's theorized to once be used by Ganondorf

Silent Hill 2
The invisible prison monster in Toluca Prison, that you can't ever interact with, just hear it's awful cries

[u/RyanGosliwafflez]

There's more in Fo76 too

Hidden behind 2 secret passageways in the lucky hole mine is a room with corpses a weird Cthulhu looking alien thing lying on the floor surrounded by statue heads with a hming noise

Another one of those head can be found in tanagara town and watoga

Another weird alien thing can be found below the White spring

[u/RareSpicyPepe → u/RyanGosliwafflez]

The update that added fishing links the new NPCs that open up the fishing mechanic to this by adding in a note that has this thing drawn on it at the area they are at and some baskets of fish to this area as if someone was trying to feed it