As previously noted, pretty much any hex value from 000 to FFF can show up as a P-25 NAC. Now the question of weather or not they are "valid" brings up a point.
On the GRE radios, I have noted some oddball NACs that show up occasionally. For example, a certain agency will use a NAC of N301. I will note that the radio will consistently show 301, but suddenly on one transmission, it may show a completely different NAC. But just on once, and the rest of the transmissions show the normal NAC.
I have not really noted this behavior on the Uniden XT radios yet.
So the question of a NAC being valid, if it displays a particular NAC consistently, then it is probably valid.
- Chris