They aren't really "secret passages" but they are definitely underutilized, and sometimes they provide a useful shortcut.
At the northwest corner of Building 50, there are two doors. One leads to a stairwell that takes you to the second floor. That's the one everybody uses. The other door is a service entrance that takes you to the cafeteria. If your office is on the second or third floor in the northwest corner, it's faster to use the service hallway to get to the cafeteria than it is to walk to the core of the building and take the main stairs.
There is a service tunnel that runs from the first floor of Building 86 (entrance next to the first floor cafeteria elevator) through the loading dock to the Central Garage, where you can continue to Building 85 or 84. This is not really any faster than the regular route, but it does have the advantage of being underground and mostly indoors, which is a major benefit when it is cold or raining.
What is your favorite secret passage at your workplace?