A few years back, retired Imagineer and now Disney historian Tom Morris uncovered something truly long forgotten and presented it as part of his lecture tours in 2019. Tom was kind enough to share with me further details and photos pertinent to the subject at hand, so I and you owe Tom a note of thanks.
When they started pouring the concrete for the façade building of the Disneyland Haunted Mansion in 1962, they naturally poured the foundational, perimeter walls in accordance with what they thought at the time the attraction was going to be, which was a twin walk-thru. As we know, the two elevators were for two back-to-back, essentially identical walk-thru adventures, at the end of which guests would ascend a set of steps to an enclosed graveyard area, where they could look around among the tombs and stuff for awhile before exiting through turnstiles.
The two enclosed areas eventually became a queue area on the south side . . .
What you may not have known is that in 1961 they were still playing around with several options for getting folks up from the basement level and into the graveyard spill areas on either side. At the time they poured the foundational walls they thought they might have THREE exits on each side. In this photo, looking across the railroad trestle that will eventually be a tunnel covered in earth, transformed into the berm behind the house, you can see two openings in addition to the main opening, the one that eventually led to the "chicken exit" in the graveyard. (That one is boarded up in this shot.)