I was talking about searches on the FCC ULS, not RRDB. If you tried to search for 774.34375 in Arizona on the FCC's site, you would've come up empty because the state is not required to license the frequencies/sites individually that are already covered under their statewide SL license.
When searching for System ID 15A on the RRDB, the Yuma system pops right up. You probably tried to search it by using the leading 0 or trailing h that your scanner displays.
Searching 774.34375 in Arizona on the RRDB Frequency Search by State returns the Yuma system as the only search result. A Trunked Frequency Search of Project 25 with that frequency also returns the Yuma system in the search results.
I'm surprised you had trouble finding it using the RRDB searches.