I worked for UT for a while as a student and talked to my boss about the tunnels.
The tunnels are not "secret" but they are definitely off-limits. Students found there would usually be expelled. The tunnels access nearly every building on campus through a secured door on a basement level. Currently a key, a keycard and a pin-code are required to legitimately enter the tunnels.
There tunnels are also, as reported, gated frequently. To get from one area of the system to another would require a set of keys that only 3 people on campus have: Manager of Campus Maintenance, his assistant, and Manager of Campus Security. The manager of each building has their own keys but can only access their area of the system.
I am told that the tunnels run to non-UT buildings that used to be part of the campus, but I am not sure where.
Also, after Tynan's escapades (I think as a result of them), cameras and alarm systems were installed.
Anyone wishing to repeat Tynan's tour will most likely be apprehended quickly as there are cameras and motion detectors in there now.
Not to mention the dozens of secret cameras hidden around campus. (If you are close enough to touch a fountain or statue, you are on camera... smile.)