Halo 5's Guardians May Have Been Lurking In The Shadows All Along
The Guardians are not a race of the Covenant, a subset of the Forerunners, or part of the Flood, but they have been present in the Halo universe since Combat Evolved.
The Guardians are responsible for all the accidental or mysterious unexplained multiplayer deaths in Halo. Whenever a kill occurs in a match that can't be credited to a player, it is attributed to the Guardians. The Halo Wikia cites a number of examples of kills being awarded to the Guardians.
Death by Guardian can occur when objects affected by physics take out a player, or by the train in the Terminal multiplayer map, the Brute landmines in Sandtrap, or as the image above showcases, the turrets in Snowbound that kill players attempting to go out of bounds. Deaths by Guardians are even tracked among your Halo statistics on Halo Waypoint with the phrase, "Killed, once again, by the guardians."
Will Halo 5: Guardians develop some lore around a mystery that is literally identified as a question mark on Halo's official website? Or is the name just a coincidence? Hopefully we'll find out more at E3.