how to handle on window.close()

Hello.

I have a requirement where a user (say manager) has a facility to see the details of his employee. THe problem which i am facing is

If a user (employee or line manager) has opened a PEP form and he is closing the browser without the log off functionality, the PEP form needs to be unlocked.

If the manager has more than one PEP form open, all forms need to be unlocked.

I have the a method in place to unlock the PEP records but how can i capture the window close event?

Thanks

RK