I haven't had anything to repair in a while because it's been slim picken's around here LOL, but my neighbor called me yesterday. He said a light was burned out on his alarm system and could I replace the "bulb". The alarm system does work properly.
I went over to check it out and told him I didn't think they were bulbs but were LEDs, and that it would be surprising to me that one of the LEDs could be "burned out", although anything is possible. I did indeed observe one of them not lit whereas on the secondary board in the house it was lit - "System Ready". Anyway we powered off the system and proceeded to remove the panel from the wall. When I pulled the panel out we found behind it a bunch of mouse droppings. We concluded that the panel was nice and comfy in the winter and made a good place for a mouse to sleep. I took the board home and said I'd get back with him.
I applied a small external voltage to each LED including the suspected one, but in all cases they lit up just fine. I have posted pics and will proceed to the next step but you are free to speculate on what
might be the problem ...