a follow-up:
I wanted to see if I could find these files, or at least the folder that contained them. So, I did a search on invisible files, and yep, found 'em (in a folder titled "504" inside an invisible ".trashes" file.
My first reaction was relief that I was at least able to see them, but, "of course", still wouldn't be able to trash them. Why "of course"? Well, they "belonged" to another user, on another machine, right?
Funnily enough, I was able to open the 504 folder and see the files. I could trash them from the 504 folder, and I could even drag the entire 504 folder (using the toolbar icon) into the trash. Empty the trash, and my lost space is mine again <phew>
Took an hour or so to come up with the idea, and to sort through the over 40,000 invisible files that my search found. I wonder if there is a quicker way (and one that's less risky too, I was afraid of deleting something important by mistake).
Let's see if my machine works ok in the next few days :-)