Best bet for memory leaks is to run it in valgrind. Something like:
  xfdesktop --quit;
  G_DEBUG=gc-friendly G_SLICE=always-malloc,debug-blocks valgrind -v --leak-check=full \
    --leak-resolution=high --num-callers=500 --track-origins=yes  \
    --read-var-info=yes --show-leak-kinds=all \
    --log-file="$HOME/Desktop/xfdesktop-valgrind.log" xfdesktop &

Then trigger the leak and gracefully kill xfdesktop with xfdesktop --quit. Note: it will be super slow running in valgrind but once you have xfdesktop-valgrind.log file feel free to attach it and I'll take a look. I also fixed a memory leak a couple days ago if you're running the 4.13/git master stuff.

Info from here: https://bugzilla.xfce.org/show_bug.cgi?id=12805#c2