This is less about a particular genre and more about a feeling. Otherworldly records. Records that stubbornly adhere only to their own logic. Records that sound like they're hiding something, but you're not sure what. Records that sound like they have elaborate backstories that you could write entire books about, but you'd be hard pressed to write much of anything if you tried.
It's not simply about the musicians being anonymous and therefore enigmatic, it's about the works themselves giving off this atmosphere. I think it's a trait especially prevalent in experimental music, these records stay with me longer than the ones that "just contain some tunes". If you know what I'm talking about I'd like to hear which acts or records give you this feeling. They also force one to listen differently, perhaps more intently.
Here are some initial examples:
The New Blockaders "Changez Les Blockeurs" . The perfect example. Impossible to describe, nothing they've done since has sounded this baffling. You think you have a grasp of the environment you're in, then your perception gets flipped on its head again and again.
Organum. David insists he's just making some sounds. When I see his artwork and read his titles I can't help but feel he's lying. Magnetic.
The Skaters. Clark and Ferraro's universe of garbled new age has long fascinated me. I'm still none the wiser as to what they're on about. Consistently otherworldly. Excellent tapes, they also had a truly unique xerox aesthetic.
The Gerogerigegege. Goes without saying. Although in this case it's hard not to talk about the mythos!