Well, I don't know how to ask you this, if it ever happened to you.
And it's from hearing sounds in your song, mix, etc. That are not there, but the acoustic effect of the mix or I don't know what, makes you hear it.

For example, now I'm mixing a song that has as main riff a guitar, that repeats a melody and in the back it has another guitar that does the same but in its third note.
Well, I was getting to the middle part where the song stops and leaves a space of about 2 seconds, and in that "silence" and the sustain of the music, I hear a guitar melody, which is not there. I don't know if you understand me, I hear it mixed in the background, but there is nothing there.
And this has happened to me many times, hearing notes that are not there but the mix makes me notice that note, or melody and then I end up adding it.
The first time this happened to me, was when I was 13 years old and I was recording with some cassettes and I was mixing from one to another (recording to the air) it was pure noise, I didn't even know a note, but in that mix of noise it filtered as a very pleasant sound, that later I ended up incorporating it.
Do you understand what I'm talking about? has it ever happened to you?

 

It is our brain ... think of optical illusions: we've learned and experienced many things, our brain connects things with each other.
In music there are so many frequencies going on the same time, sometimes it is difficult to tell what is going on exactly. There might be a little reverb or delay thing floating around, maybe it is a certain note, then there is another reverb tail of anything right after that or something else, maybe even silence. Then our brain comes up with an idea, a connection. Voila there is your melody.

Another example: during rehearsals with my post rock band there is so much going on. Two guitars shredding 16th notes with reverb tails longer than some say is allowed, a loud bass, drums all over the place. There is more you hear at the same time, 'cause all the walls and stuff are reflecting the trillions of sounds, so it adds up even more. Sometimes you can hear "a choir of angels" (not my words, but of a friend after a gig) singing a certain melody, a melody none of the guitars are actually playing. I guess our brains tend to sorting things, organize them, giving you orientation points, etc...

Is this what you are talking about?

 

Our brains make us hear all sorts of shit that isn't there. Here's a recording of legendary producer Joe Meek talking to a cat:

However, in your case it's obviously the ghost of a long dead guitarist, attempting to squeeze in an ectoplasmic riff from the other side into that 2 second stop. I'd either fill all gaps with music or call an exorcist.

 

Yes, that's exactly what I'm talking about.
I have never discussed it with anyone. So many crazy things go through my head. Sometimes I don't know if it's my madness or if there are really things happening. So only here I dare to express them.
It never happened to me in a rehearsal. But everything you have mentioned has logic and sense. The walls and the sounds sneaking in between them. What a really cool thing to happen.
Talking about "optical illusions" and the mind that creates them. Today I heard someone talking or whispering in my bathroom. And then I put my head listening in my yard (my yard is across the street from the bathroom) and it turned out to be a dripping faucet and the sound sound sounded like two people whispering. The power of sound never ceases to amaze me. Thanks for sharing your experience.

hahaha bro. I don't believe in that stuff. Sorry. I believe that the dead no longer exist and that's it. I'm inclined to think that it's our brain that creates that sound illusion.
But if weird things have happened to me, I won't deny it. But I think it's our brain.
I was very amused by this: "I'd either fill all gaps with music or call an exorcist."

 

I'm a producer so I don't mix too heavily but these ghost melodies/sounds happen a ton to me while I'm crafting tracks, and it's in my favor, fortunately. Perhaps we're just so used to hearing sounds in our heads it manifests in everything we do

Nice to hear I'm not the only weirdo and that other people also experience this.

 

That's the good thing bro! We are several crazy people hahaha

 

Yes, it also happen to me sometimes. Most of the time it is due to some kind of harmonic resonances between different sounds and notes, and I hear it like if it was under the mix.

Depending of the composition, sometimes I add a part to reinforce it, but it does not always work as once reinforced it can clash with other elements, sometimes I let it as it is, and sometimes (the less) I try to get rid of it.

Anyways I always find it very interesting.

 

Is this a deleted scene from This is Spinal Tap?

Returning on the topic, I was once listening to a George Michael album while tripping on acid, and I could swear I heard the background vocals singing "Say F**got Say F**got"
Could've been worse, could've been the devil telling me to kill myself through Judas Priest.