Hidden song in "The Promise"
 

I apologise if this has already been discussed.

About halfway into the basement demo "The Promise", you can hear electric guitars being brought into the mix, but only for a few seconds. You can also hear them in the background just before and after.

From the brief moment you can hear them, they seem to be in the same key as the main song, but I can't tell if they're some part of the song or another song being leaked into the mix somehow.

Does anybody have more info on this?

I don't think that's a hidden song. The Promise was only recorded during the Basement Sessions, meaning they recorded it onto a small tape recorder and there wasn't a mix on these demos at all.
It could be that there was accidently something overwritten on the audiotape used for a few seconds, I guess we'll never find out.