I have been looking in the NEC and can't seem to find the code which states whether THHN can be free wired for low voltage wiring? Anyone Know?

My interpretation is that THHN is not allowed, see NEC 2005 Section 300.3, states, and I paraphrased, "single conductors listed in table 310.13, (which THHN is listed,) must be installed in accordance with any methods listed in Chapter 3", which does not show any method of attaching single wire to a structure freely.

The closest I could come with that may allow it is Article 398 Open Wiring on Insulators. This method has a few conditions that would be prohibitive to install this wire in this fashion.

In short, THHN is listed as an insulation that must use a raceway or method listed in Chapter 3.

You should review Articles 720 Circuits & Equipment less than 50 volts, and,
Article 725 Class 1, Class 2, Class 3 RC, Signaling, & Power Limited Circuits.

You need to determine which Class system you are installing, and then look at the limitations of insulations allowed, esp fire rated installations.

Why not just use LV cable?

THHN is rated 600 volts, so that is not the issue, the problem is toxic smoke created during a fire, and physical protection of the wire.

Out of curiousity, what were you considering, Alarm loop?