I think what the OP and Grapeshot are missing here is the definition of "prohibited area".
76-10-501.
Definitions.
As used in this part:
...
(14) "Prohibited area" means a place where it is unlawful to discharge a firearm.
I posted about this very topic a few fears ago, and here is my opinion. I grew up in the city of Lehi, Utah. Lehi has an ordinance making it unlawful to discharge a firearm in the city. If the city were to
POST this prohibition, say near the signs that say "Welcome to Lehi", then openly carrying a loaded firearm would be unlawful in Lehi without a permit.
Please keep in mind that loaded open carry is not unlawful in Utah, unless in a vehicle without consent, or on a public street. I am also of the opinion that 76-10-505 does not exist at all for permit holders. What I am saying here is that consent from the person in possession of a vehicle is not required when one has a permit from any State or County. Sure, the person in possession may make whatever rule they wish, but carrying without consent is not a violation of 76-10-505 if the person is permitted.
Again, going on the definition provided above, I believe this was written to prohibit loaded carry in places where discharge is prohibited, which could be a lot of places. If the OP thought that "prohibited area" meant places like courtrooms, secure areas, and so forth, then I am certain he is mistaken.
...my opinion.
Welcome to OCDO, sjmarsing!