I found my location via the
"Open Files and Ports" location in Activity Monitor mentioned in the other answer.
This lead to the location:
/Users/USER/Library/Developer/Xcode/Archives/2018-10-09/MyApp 10-9-18, 9.35 PM.xcarchive/Products/Applications/MyApp.app/Contents/PlugIns/MyAppFinderSync.appex/Contents/MacOS/MyAppFinderSync
This archive was 10 days old and I'm not sure why it would have been running. I've been deleting my app since that date, and I believe the extension was being removed from System Preferences.
I deleted the archives in that location, and the app extension no longer runs (which it was doing before). However, it's still listed in System Preferences. Still looking for a way to fully delete.
Edit: I started running a new version of the container app, from
/Applications/. About 10 minutes later, I went back to System Preferences and toggled the Extension on/off and back to on. This launched the new version of the Extension and everything appears to now be working normally.