Mystery (finally) solved, but not without a few detours. Turned out to be a mapping error situation which several hosts in my area have reported.
The detour: I previously had “free parking on street” selected, but not “free parking on premises”, and apparently the former doesn’t qualify as “free parking” in the search filter. Ok, that one is on me. I have off street parking in back of the building for my long-term tenants (it’s not nearly as convenient as parking on the street in front of the house, but whatever…), and there is always street parking available (except in winter, when it snows and cars have to be moved for plowing), so I’ve turned on “free parking on premises” and will paint a couple stripes and put up a “Reserved” sign in front of one space and call it good. 
Changing that didn’t fix the problem, however, at least not right away. Three hours after I made that change, I received an email from Monica, telling me that she was looking at my listing. So I opened up my laptop, cleared everything, opened a new browser window, put in Portland, Maine, no dates, property type=apartment, entire place, and self check-in… and, voilà! It was showing again. Not on the first page, but IT WAS VISIBLE AGAIN. 
I was having breakfast with my husband a few hours after that when I received a call (yes, an actual phone call!) from Airbnb. I don’t recall his name, but the man spoke with an Indian accent. He wanted to advise me that this was apparently a problem that a number of hosts in my area have reported, and that they were entering the latitude/longitude coordinates for those properties and that he wanted me to know they were being responsive to their hosts, etc. etc. He started giving me a big schpiel about why my listing wasn’t showing on the front page (That it had dropped in the search rankings following the initial new listing bump, etc.), but I cut him off and said I was just happy that it was showing up again, after 11 days in the dark.
I did also let him know I had received four inquiries via homeaway.com in less than 24 hours, which is more than I had received on any single day while the listing was actually showing on Airbnb. (So I know the property itself is attractive).
And, yes, Helsi, the artwork has gone up this morning, and the planters were filled before my first guest arrived, so I’m snapping new pictures after I finish re-caulking the bathroom and will put them up on the listing tonight when I get home. Unless I pass out first…
Thank you all for helping me through this frustrating time!