Also what would make one a major faction, would the Kig-yar qualify?
But to come back to the Forerunners, I suppose, that would limit it to people who might have heard those rumors but I think I have someone.
\* Some time after the capital was secure a ship emerge in the nearby space. The few Forerunners who were there when they first awoke might recognize that signature as a ship that was present at this moment and belonged to the space faring person who first met them. \*