+1 to
@slicerwizard's posts
There is nothing "proprietary" about the Site ID on either a Motorola SmartZone 3600 system, or a standard P25 9600 system; they are encoded in plain sight (no pun intended).
The fault lies entirely with the scanner manufacturers, who long ago made the conscious choice
not to incorporate the Site ID in the programming. In fact, they don't even make use of the SysID or WACN-SysID, which are also in plain sight. Instead the scanner simply locks onto the very first control channel it finds in the list of programmed frequencies that matches the system type, and away it goes.
The laughable part is that if you put most of these scanners in any sort of control channel quick-tune or "analyze" mode, almost all of them readily display the SysID or WACN-SysID, as well as the Site ID. In other words, the scanner is perfectly able to decode those parameters, but still doesn't use them when you program the system.
The obvious side effect is that any co-channel usage by an undesired site that the scanner is able to receive may result in the scanner locking onto that site, not the one you're expecting (as the OP has discovered).
At least the Uniden SDS series can have the Site NAC programmed, which
may convince the scanner that it's locked onto the wrong site. Of course this assumes that the system is setup with unique NACs for sites that share frequencies. Unfortunately, that is not always the case.