Thats much, much more common then people living in the sewers, par se, outside of really large cities with subways (like Boston or NYC/etc.) But its a great trope.
One other source...though to my knowledge I've never heard of people LIVING in them...almost every former 'boom town' in Pennsylvania has extensive tunnels under it. Butler, PA, for instance, has huge semi-flooded tunnels where guys sneak in to fish. (I've never been there, and I think they are IDIOTS, but guys go there and fish on occasion.) New Kensington, PA, has a lot of old WW2-era aluminum plants connected by extensive underground tunnels. They don't connect with the sewer systems, and flood after it rains, but I've seen tons of people go in there for various practical reasons. (usually related to utilities, subsidence, flooding, or somesuch.) They are easily big enough to walk in. Parts of Pittsburgh still have coal mines under them dating to the early 17th century... and thats entirely ignoring the plethora of abandoned train tunnels which you can find in pretty much every part of the NE, including dense urban areas. Even NYC has those.
Thats one unfortunate side effect of the 'mines as exotic dungeon' part of the grid. Probably it would be fair to have a number of towns (past a certain size) - have disconnected, long-abandoned mines or tunnels underneath them. It really isn't uncommon in coal country or the rust belt in general. Your looking more at the southern and western parts of the 'northeast' - but those small to mid-sized town areas are more representative of the game as it now is, anyways, and could give plenty of inspiration for areas.
Like, off the top of my head for areas/rooms/etc.
1. Stormdrains (with variants that have collected dead zeds and other such things)
2. Decades to centuries abandoned mines. (Variants: Partially Flooded)
3. Utility closets (Variants: Electrical, Plumbing, Cable).
and so on. I mean, this is probably overboard and I'm sure you'll find it silly, but it is actually kinda realistic for the area, if you really wanted to cut back on the 'walking sewers' and find more realistic alternatives, as several have commented on.