I tried to a rename a rendered clip's file name. The attempt was refused because the file was open in Kdenlive. Except that, as far as I could see, Kdenlive wasn't running. Task Manager, however, showed six kdenlive.exe background processes. What's doing this?

This occurs under Win10.

Make sure you downloaded the actual version for windows: 17.04.3.
You can get Kdenlive here (pick one that ends in .7z):

https://files.kde.org/kdenlive/release/

There isn't an installation for Kdenlive - you simply unzip it to wherever you would like. You double-click on kdenlive.exe (or create a shortcut) in that folder and the program starts.
If you need the codecs for full FFmpeg binaries, follow the instruction on download page for windows “Installation (requires FFmpeg download):“ and copy all mentioned files into folder “Kdenlive-17.04.3”. https://kdenlive.org/download/
With Kdenlive-17.04.3 you can close the program by klick on the top right “x”.

There is a bug in the Windows version older than 17.04.3 of Kdenlive; so that if you crash or close the program, it keeps running in the background.

After close Kdenlive; press CTRL-SHIFT-ESC to open the task manager. Look for the following entries in the list:

kdenlive.exe <-- there may be more than one
kioslave.exe
dbus-daemon.exe

Highlight each one and press the DEL key or click on 'End Task' to stop each process. Now, you should be able to run Kdenlive again.

I've been using 17.04.3, dropped into the Programs Files folder (BTW, is 17.04.3 32 or 64 bit?), the ffmpeg library is AFAIK current. It appears that using the close (X) in the upper right is more likely to leave a background process than exiting File -> Quit or CTL+Q

i can confirm that on latest build of KDEnlive .. (windows 10 64) Kdenlive-17.04.3-w64.7z
here a screenshot after closing KDEnlive main executable from "X" on top of window.