I would try double checking the info under. . . >SketchUp Preferences >Files
…and I think that should bring up the option boxes for routing file location.
Just in case it’s needed Backup files have the .skb file extension. Which gets renamed to .skp as the the reactivation step.
It might also be worth it to try a Windows based Wild Card Search for… *.skb
(e.g… include the asterisk as the sole stand-in for the file name)