I assumed (erroneously, I think) it was from the 2nd Floor CO detector that was alarming simultaneously with the other sensors sharing the COM port. Your question made me think about the wire itself, and I realized the 4-wire CO detectors in my house actually use green and white wires (and red/black for power).
So this thick sheathed brown wire you see on the troubled COM (and also another identical looking wire connected to the lower COM terminal where only 2 wires are connected) is most likely not for a CO detector.
It might be for motion sensors (maybe?), although I don’t know how to test for them. When I was checking the terminals by disconnecting each wire from the terminals to see which sensor would trip, the circuits with the thick wires (terminals labeled “Zone 6” and “Zone 2” on the top expander) did nothing (no indication on the keypad, no sound) when disconnected from the panel. I thought it was because I had deleted the 1F and 2F CO sensors from the panel.
(By the way, I tried to learn those CO sensors back into the system last night, but I couldn’t trip them. They are still at their EOL state, as I wanted to figure this out before ordering replacements. Could that be the reason they wouldn’t learn into the panel? I pressed the hush/test button for more than 10 seconds and nothing happened.)
My basement CO detector (terminal labeled “Zone 7” on the main panel) tripped immediately when I disconnected, and the wires for that were green and white.
You made me realize that untested assumptions can lead to wrong conclusions.
Yes, once I figure out what the think brown sheathed wires are for, I can try this. But now I think the 1F and 2F CO detectors are probably connected to terminals on the main board. (I have yet to test Zones 1-6 on the main panel.) I’ll go through all of them to complete the physical map of which terminal is for which sensor.
Thank you.