What are files named something like Filename.gnucash.tmp-a00396

[Dennis Shimer]

I understand the backups and the log files but recently I had friend with a
gnucash file just disappear. One day it's there, the next it isn't I'm not
done helping him trace down what actually happened but there was a file
with the extension .tmp-a00396. I know he is using a fairly recent version
on Windows 7 and the file looks like something ver near the last saved
version of the .gnucash file.

What is this file, where does it come from, and how likely that it is in
fact a complete copy of what he was last working on?

Many thanks.
Dennis