However, there may be tunnels where you don't want students (for example, entrance of water mains, sewers, electrical power, shipping and receiving for materials or for the kitchens or...). These would not be shown on public maps of campus.
They are not "secret" in the sense of being disguised as something else. They would simply be protected by locked doors and signs of "Do Not Enter".
In the same manner, there may be storage space where you do not want students or general public or even teachers to enter. For example, the refrigerated storage room for the kitchen food. Only some kitchen staff would be allowed in there. Because these spaces would normally NOT be shown on public maps, they might be "secret" to some people.
In one place, we had a shooting range "hidden" under the swimming pool deck (not under the pool bottom, but next to it). The range was for the use of the local police (not for the students nor the staff). It was not shown on public maps of the building and had no signage indicating how to get there. Either you knew or... you had no business there.