EDIT: Mystery solved, sort of...
I'm ripping CDs, so working in the Recently Ripped view. In THAT view, Delete will immediately remove the record FROM THE VIEW, but ONLY from the view. It's sort-of handy behavior, but inconsistent with how MC behaves in other views.
When I navigate to another view and look at the ripped album, the "deleted" tracks are there.
I still hope for a database record undelete option, though I now have an easier way to find the "lost" track. Apologies for not recalling that Recently Ripped behaves differently.
To recreate what happens:
1. In Recently Ripped view, starting editing a column value, such as Name.
2. Go to the start of the field and press Delete, or the end and press Backspace, a couple of times.
3. Midway through, press Up Arrow. The cursor jumps to the record above.
4. Pretend you don't notice, and press Delete or Backspace again -- the record disappears, no confirmation.
Of course, in Recently Ripped, Delete will do so without prompt whether editing anything or not. It's just that editing is where the Delete/Backspace key might be commonly pressed.
Oops... testing this just triggered the bug that changes the record's filename inappropriately. But that's not related to the situation I'm reporting.
BTW, restoring from a backup is too brute force for this type of problem, which usually happens when I'm doing a bunch of ripping and tagging (450+ tracks this time) and restoring a backup that is more than a few minutes old would likely wipe out a bunch of good work.