As suggested, I retraced my steps during this job, but found nothing I'd touched related to the mysterious washer.

So, I reached back further in time, and remembered that I had recently removed the B11/2 Coolant Temperature Sensor to determine its part number for finding a potential replacement. I haven't confirmed the match physically yet, but the size of the washer and sensor from memory suggest that they belong together. Plus, when I replaced the sensor I remembered thinking that it was odd that the orientation of the two top jacks was slightly different (and even feared that I had overtightened it).

Here's an image showing the sensor, where it lives on the engine and the faintest hint of the loose washer from photo taken the very next day. BTW, the OP photo was staged, as I had extracated the washer to look at it, then thought to take a pic.



Thanks for your help! I'll be getting that washer back on B11/2 ASAP. I consider myself lucky that the washer didn't fall into the Infinite Void.