They are not all sealed.
I got in from underneath the Dude Rancher Lodge a while back. A friend worked at the front desk, the entrance is not hidden at all, right in a hallway. Go in the front door, make a left, then another, then down some stairs to "storage". They have a ton of old chairs and such down there, but beyond that mess it heads straight down town. Pitch black, no skylights like you see downtown, they were surely paved over.
I know there are a few other buildings with access too. Nothing connects anymore, they're all dead ends, bricked up between almost every building. But, the one at the Dude Rancher goes further than I dared to explore, I'd guess it stops at Alberta Bair. The tunnel also continues to the North-East, towards the hospital. I was told it's sealed a few hundred yards up that way though.
Laramie, Wyoming has similar, but not bricked up (15 years back). I ended up in long forgotten basements, packed full of antiques, there's even a couple old walk-in hotel safes. Rooms that hadn't been visited in 50 years, it appeared the building owners probably didn't even know of the spaces, as someone had sealed them off years ago. They end/start in the old train depot tunnels. There has got to be a gem or two like that under Billings somewhere.