While working on a Firefox extension, Firefox deleted a file I was using, "overlay.js". It was gone, but fortunately, a few minutes previous, I had manually deleted a temporary backup file that my text editor was using, "overlay.js~".
Since I had manually deleted it, it should have been in my Trash Can, but it wasn't, probably due to the "~" at the end of the filename which indicated that it was a temporary file. Even "Show Hidden Files" didn't make it appear.
However, by going to a command prompt and viewing my trash folder there, it appeared:
So I'm not sure why the Trash Bin didn't show it, but it was there.