"Marble Armrest of Xerxes" is up now.
Guards Encountered: Tomb of the Headless Kings, Quicksand Bog
Number of Unencountered Guards: 1
Rooms not Entered: Secret Password, Shrine, Ancient Warriors, King's Storeroom
Notes: Nobody had any specific locations as to where the last guard might have been.
My Thoughts: The half pendant was located on the door leading from the Jesters' Court to the Dark Forest. Why Noelle signals to the center Jesters' Court painting, I will never know. But since all of the rooms prior to the half pendant were entered, we know that there were two guards beyond the half pendant. Furthermore, the final guard had to be on the top floor.
-- There was no way to avoid the Tomb of the Headless Kings because it was the only room that opened from the Pit. Also, there wasn't a key in the Dark Forest, so Noelle was forced to encounter the guard in the Quicksand Bog. Since both of those guards were forced, the final guard probably could have been avoidable, even though there were a few runs (particularly 1.5 pendant runs in Season 3) where all of the guard encounters were forced.
-- Since this Center of the Rooms run featured a clockwise path, I don't think the King's Storeroom would have been accessible from the Ancient Warriors. Therefore, the only other place an avoidable guard could have been was in one of the Ancient Warriors. The Ancient Warriors held guards in a number of other Center of the Rooms/King's Storeroom runs that featured a clockwise path. Also, we know that there were instances where a guard was stationed in an inactive armor (Chandragupta and Ivan the Terrible), so it would not have been far-fetched to have a guard stationed like that for "Xerxes." Since the inactive armor guards were stationed in the armor nearest the room the contestant entered from, I would guess that, if the guard had been in the Ancient Warriors, then it would have been in the far-left headdress.
-- If all three guards were meant to be forced, then I think that the final guard would have been in the Shrine. It was common for the guards to be spaced out every-other-room, or just about every-other-room, and that pattern was seen in most episodes. The Shrine did not open from the Quicksand Bog, so the Secret Password would have served as the gap room between the Quicksand Bog guard and the final guard in "Xerxes."