I was looking for a recording this morning in the hard drive, and came across some old episodes of a show that were deleted long ago. In the DVR recordings section there is no record of them, but there are 4 extra files (two of which seem to be a duplicate). I'm sure I can manually delete them, but any thoughts on why they got left behind? 
Strange, not sure what happened. You could look at the full log (mac menu bar, tv icon, view log) and try to see if there were some errors when it tried to delete those for some reason.
Looks like my log is only going back two weeks, don’t see any errors in that time frame. These extra episodes seem to be from back in 2017.
I can manually go through the Finder and see if other shows have extra episodes remaining, but wondering if there could be some regular database maintenance to reconcile DVR listings vs folders?
In the meantime I assume it’s safe to manually delete these?
Yes it is safe to delete.
If you access the Log via the menu bar, it should be complete. Unless you reinstalled the DVR at some point.
Yeah, I did reinstall a couple weeks ago when I put a new SSD in the computer.