No such luck. I began to search around, with no success. The ring is titanium, otherwise unadorned, and very lightweight. It may have landed edge-on and bounced over the concrete curb into the low ground cover; however, it's not heavy cover, and I would have expected it to show up easily. I searched for about 20 minutes, moving outward to check less-likely places (including inside the doorway). I found nothing, and I'm very annoyed. I'll look again later this afternoon when the cars parked at the foot of the stairs have left, in the hope that maybe the ring rolled down under one of them and I couldn't see it when I looked under them.
There is a spare ring up in Oregon, and it's slightly smaller and thus less likely to fall off (this is not the first time the ring has slipped off my hand), but still, I'd rather not have lost it.
Update, 16:40: Found it! I made up a couple of signs saying "LOST WEDDING RING" and giving the particulars and contact information, and as I was taping one of them to the door, a woman from the company downstairs came through. I explained the situation, and she made sympathetic sounds and looked around. Then she said, "Wait, I think I see it!" and reached down and plucked it off a small bush about two or three meters from where it fell off my hand. The ring must have fallen off my hand, hit the opening door, rebounded off the concrete over a railing and onto the branch. You could not have done it on purpose if you'd tried. What a relief!