I was going to suggest a "Sends" issue as Brundlefly did but if you have not recorded ANY vocal tracks into this project and there is no vocal track at all... then this is rather strange indeed.
There is no possibility of ghost track tape type bleed in the digital realm AFAIK.
So... what I am thinking is this is your rig picking up radio transmissions somehow.
Is it ALWAYS the same vocal, always at the same points (as in if you hear a faint vocal saying "Hi, I'm a ghost vocal" and it says the exact same thing every time at the exact same spot as if there were a track somewhere). If so then that's kind of... impossible. BUT maybe you have hidden a track using the Track Manager that is still active and just forgot you recorded a vocal.
That is unlikely but the only way that could happen (I think).
For the radio interference stuff then the vocal would change all the time (so you're picking up a radio station or something due to bad grounding). Are you using monitors or does this occur through your headphones as well?
If it's through the monitors check on your headphones to see if it's there as well. Monitors may pickup signals on their own (maybe) but I don't think headphones through an interface could (or maybe the interference is COMING from the interface).
To me though for such interference the most likely culprit could be through your guitar and guitar amp. Do you have the Input Echo engaged on a track that has your amp mic'd up to or is accepting a live feed from your amp? Find out and turn off the input echo or just turn off the guitar amp to see if that gets rid of it.
Sometimes mixer's and PA's can do the same thing. If you have those things hooked up try turning them off or mucking around with them.
Alternatively... maybe someone is simply playing music in the next room or outside or in your building.
And I hesitate to say this because it might be mildly offensive but are you sure it's not in your head? Sometimes when we focus too hard on the physically audible those little melodies and what nots that rattle around in our brains get mixed in. That's actually how I write is by trying to "hear" those sounds and they generally stay in my head (and if they don't I know they aren't actually there... I hope lol) but altered states can blur that distinction and all of a sudden we are thinking we are hearing things that our ears truly are not.
On the bright side maybe you subconsciously wrote a vocal part and are willing it into existence. lol
I'd be quite interested to hear what the answer is.
Cheers.