For your purposes, consider the panel's Addressable Signaling Line Circuit (SLC) output to be a power supply output. The panel's SLC output is what provides the power for all of the addressed portions of the SLC's detectors and modules.
The SLC power supply is usually a lower voltage power supply than the batteries. It also is a very weak power supply and can be shorted out extremely easily.
The SLC carries data between the panel and all of the devices like detectors and modules. When you measure the SLC voltage with a voltmeter, the data makes the voltage measurement very unstable. Don't worry about the unsteadiness of the SLC voltage, it supposed to be that way.
Right now, when the building's SLC circuit is connected, the SLC circuit is shorting out the power supply.
The problem you're seeing is that even though your ohmmeter says that there isn't a short on the SLC, the panel thinks that there is.
- There could be a bad component inside an addressable detector or module somewhere in the building. Even though the ohmmeter can't detect the bad component, the bad component can short out the weak SLC power supply.
- Water could be on the wires, or there could be some other damage to the wires. Even though the ohmmeter can't detect the water or other damage, the water or damage can short out the weak SLC power supply.
- A detector or module could be wired backward to the SLC. Even though the ohmmeter can't detect the backward wired detector or module, the backward wiring can short out the weak SLC power supply.
Use the Panel Instead of Meters
In this case, about the only way to find the problem is to assume that there is a short on the SLC, somewhere in the building. Also, assume that your regular ohmmeter won't help you.
To find the short, instead of using an ohmmeter, use the SLC output from the panel. When the SLC's voltage shorts-out to zero volts, the SLC is telling that short is still on the wires. Then, start going around the building disconnecting wires and checking the voltage on the SLC until the short goes away.
It's going to take work, and you will have to think about where the SLC wires are going, but eventually, you'll find whatever is shorting out the panel's SLC weak power supply.