ExMos seem to have this fascination with tunnels underneath SLC. News flash: there are tunnels underneath every major city. It's how utilities and steam heat gets into all those high rise buildings.
I can get into 7 high rise buildings sited on 6 acres from my parking garage in downtown SLC. The parking structures underneath City Creek Mall and all the adjacent office buildings, Temple Square, Eaglegate Condos, and the COB cover more than 40 acres and all have at least pedestrian access to each other because they are all adjacent to each other. It's not all that amazing, people. Y'all can stop oohing and ahhhing about the "secret tunnels" under SLC.
In the block I live on, you need a key card to access some of (most of?) the elevators. I have a key card that gets me into three elevators, and one of the elevators (that I know of - I haven't checked the others to hotels) is accessible to the general public. So nothing amazing about limited access doors under City Creek/COB/Temple Square either. It's a pretty basic security feature. You can't get to COB or Eaglegate without a card.
BTW, the City Creek parking garage is 4 levels deep, though I don't know if all levels are accessible to the general public. I assume part of it is reserved for office building employee parking. Last time I checked, the first two hours of parking there are free, so if you're making a quick trip downtown, that's the place to go. Tip: remember well where you came up to street level. Otherwise you may have a difficult/impossible time finding your car again. There are "you can't get there from here" situations, even with all the secret tunnels. :)
I talked to the HR manager of the City Creek Garage about a job and we both decided it wouldn't work. But he told me some of the things about the garage.
One of which is that when you're in that garage, driving, looking for a space, they don't want you to know what's above you in the mall. So when you take the escalators up, you come out and if you're not near the store you wanted to go to, you have to pass lots of them and be distracted.
I know I've been in there maybe 10 times since I came back here in 2015, and always have to take a picture of the parking signs with my phone to make sure I know where I'm going back to.