Regarding your question about monitoring local Sheriff's Office (SO) and Police Department (PD) radio traffic using unregistered two-way radios, the answer is generally no. These radios are typically registered to a specific system, which means that unregistered devices, such as your 878, won't be able to pick up the traffic.
I attempted to monitor with my own 878 and observed that while it indicated someone was transmitting (keying the radio), I couldn't hear any actual traffic. Conversely, my SDS100 scanner was able to receive the communications clearly.
Based on this experience, it appears that you cannot use a two-way radio to listen in on these communications if the radio is not registered with the system. This is likely due to the system's design to secure communication channels for authorized users only.
The 878 wont pass audio because the system is using using RAS. RAS is variable that you create when programming, that has to be selected in each channel you are using RAS.
It appears the system is conventional. If it was not running RAS, using a Motorola DMR radio, properly programmed with CC, TS and TG for each frequency, would work just fine, and as long as the channels are configured to be receive only, your radio would not transmit. Your 879 would also be able to receive the system.
It is not a Connent Plus/Capacity Max/Tier 3 trunking system. The radio doesn't need to register or affiliate to receive and pass audio.
The ONLY reason you can't monitor(unless it's using encryption) is because RAS has been enabled. Proper DMR compliant subscribers will not unmute because of how RAS hashes the headers of the packets.
There may be some cheap Chinese radios that will unmute RAS enabled systems, but I don't have specific knowledge of which radios, if any, can do that. Scanners will unmute though.
