I replaced the chain on the beast today, the old one vanished on the way home from work on Friday while I was on the bridge. I heard it go as I was nearing the top of the arch of the bridge, so I least I got all the way up the hill. My guess is it's somewhere in the Fraser River as it wasn't to be seen anywhere along the route behind me. It gave me an excuse to take the other bike out for a run and go over the route a few times to see if I could be that lucky though. So, having replaced the chain it's happy to start up and run for me again and the chainguard's back on.
Now it's time to go back to the garage and clean up after myself and make sure everything's ready in case the weather's good this week. And cruise around the block for a bit, most likely.