That file is easy to reproduce--just save a workbook.
But the bad news is you don't see it (or notice it) unless something goes wrong.
If you open a giant file (10-15 meg) and then open windows explorer and go to
that folder, then do a save in excel. Quickly swap back to windows explorer and
you'll see it get used.
(I think in some versions of windows, you may need to hit F5 to refresh the
windows explorer window. Hit it a couple of times if you don't see the filesize
(in detail view) grow.)
The .xar is different. That functionality was added in xl2002.
You can see your folder that is used by using excel's
tools|options|save tab.
But this is the file that's saved every xx minutes (user selectable). If excel
or windows crash while you're making changes, then this .xar file stays in that
folder.
The next time you open excel, it'll check that folder for any of those files.
If it finds some, you'll see a prompt to determine what you want to do next.
If you see one of the .xar files hanging around (or just edit a workbook in
excel), you can copy it to a different folder, rename it to .xls (or just
file|open the .xar) and it'll behave just like an excel file--'cause it is one.
I use this as my folder to contain my .xar files:
C:\WINDOWS\Application Data\Microsoft\Excel\
I think that's the default location for me. It's buried deep enough that I
don't go looking here. And You saw the funny named file in the same folder as
the original workbook--so I don't think you saw the .xar file.