Run a script (server side) when FTP user logs off

Question

I'd like to run a shell script on an FTP server when an FTP user logs off.

Imagine:

1. Ftpuser logs in, uploads a file to an inbound-only directory and logs off.
2. Ftp Server detects logoff then runs FileCheckMove.sh
3. FileCheckMove.sh checks/sanitizes any files from inbound-only and moves them to an alternate location.

What's the best way to detect ftp user logoff and initiate the event?

• I've considered simply auto running the script every few minutes, but I don't want to attempt FileCheckMove.sh on a file that's open during its upload to the server.

This is Debian 9 stretch running ProFTPd.