who/what has an Outlook .pst file open?

I have an outlook .pst file on a network drive. I use it to store specific e-mails not related to work , so it sits as a personal folder off my mail profile when I attach it about once a day to housekeep.
Today it tells me that someone else is using the .pst and for me to close all apps that would be using it.
I have shut down every thing I can think of – word, outlook, anti virus, system tray stuff and many more to no avail.
I am the only one with permissions on that folder on the network and have full control to the .pst.
I cannot copy, rename, scanpst, hex edit or anything as it still remains in use by something else.
Can any one help me work out what/who has this .pst file ‘open’?