Calvin attempts a diagnosis.  He went through everything, dishwasher, disposal, seals, etc., etc., nothing.  Our plumbing is solid, nothing wrong with it.  So the thought is the extensive plumbing work our upstairs neighbors have been doing is likely the source.  So, it's a watch and wait.

And no, this mysterious plumbing issue is not the reason why I've had to stay behind an extra week, just a bonus hassle.

Sigh.