Here is a update on my X10 outlet that stopped responding. It was actually a Leviton 6280, which is the version with the pass-through to control other outlets downstream.
I checked it again today in the garage, and there was no response – no click, nothing. So I swapped it out with a replacement, and got the click … click in response to ON … OFF. Great, problem solved!
Then I opened the old 6280 up on the bench to see what was wrong. There was no evidence of any damage. Nothing was burnt, and no electrolytic was expanded or leaking. So I connected up power, and was surprised to find it responded perfectly to commands again.
I don't have a clue why it was dead in the garage. Maybe its controller was locked up and it just needed a power cycle? Maybe the physical stress that occurred during removal and opening the case corrected an intermittent? I had spun the code wheels, so it wasn't an intermittent there. Since it appears to be fine now, I'll save it as a spare for the one that controls the outlets in our media room.
Jeff