Hello I am familiar with DC's subway system and I am somewhat skeptical of the "hidden subway station" theory and very skeptical of the "Dupont Underground" theory.
First, the Dupont Underground theory: The tunnels are not directly related to the Metro. They are the remnants of DC's streetcar system that went obsolete in 1962. But I don't believe that those tunnels get anywhere near Comet because of geography. The streetcar tunnels are specific to a few blocks around Dupont Circle. They would have been aboveground most of the way to and past Comet because there's a geographical feature known as Rock Creek that lies between Dupont and Chevy Chase, which has cut a wide and deep canyon spanned by the Taft Bridge https://en.wikipedia.org/wiki/Taft_Bridge. (For reference, Comet is located well past the north (left) end of the bridge and Dupont Circle is located well past the south (right) end of the bridge. If you look at the pictures, I think you'll agree that the early streetcars did not tunnel all the way under that gorge. (And see the streetcar picture on the Taft's companion bridge for East/West traffic over Rock Creek, the Duke Ellington https://en.wikipedia.org/wiki/Duke_Ellington_Bridge).
For these reasons I do not believe that the Dupont streetcar tunnel gets anywhere near Comet.
Now, to the existing Metro tunnels: I have ridden the Red Line many many times and you can definitely note when the northbound train makes the bend to move from running under Connecticut Avenue to running under Wisconsin Avenue. It is not impossible that they made another branch that kept going under Connecticut, but I have never heard of one. But the important thing to note about any potential tunnel/station in that spot - it would be very, very, very deep. The stations on either side of Comet, Tenley to the north and Van Ness to the south, require long elevator or escalator rides to descend into them.
That being said, I recognize that DC is a city of secrets and I wouldn't discount anything - but I thought there should be some better understanding of the geography to inform opinion.
Finally, I was trying to fine the exact depths of the stations and couldn't, but here's a site that deals with some of the history and engineering issues. https://chnm.gmu.edu/metro/eng2.html
Thankyou for the local info and links!
If this was any other city but DC, I wouldn't put so much weight into this theory - but with it being so close to the known/public underground tunnels + rumoured to be next to a huge private govt/military system I thought the possibility was worth mentioning.. Thanks again :)
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Footnote 5 http://ghostsofdc.org/2013/10/01/never-built-metros-bridge-rock-creek/ at this link https://en.wikipedia.org/wiki/Taft_Bridge has some interesting info about the Metro engineers' original plan to have the tunnel at Rock Creek be somewhat shallower and come outdoors and actually PIERCE the columns of the Taft bridge. So the subway trains would emerge from the canyon wall, travel along the Taft bridge but underneath the main roadway, and go back into the canyon wall on the other end. This would have allowed the tunnel to stay a bit more shallow.
And as a Metro user, that would have been SO COOL.
But the important factor here was that the National Park Service, the federales who run Rock Creek Park, said hell no. So all the stations that serve the West side of the Red Line had to burrow that much deeper to get under the Rock Creek gorge.
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