Odoo 12.0+e(Enterprise Edition)

Hi, I would like to add some extra functionality when the 'Discard' button on the mail.activity form is clicked.  I'm having difficulty locating the method/code that gets executed when it's clicked, if anyone knows what file this is located in and could let me know, that would be awesome.

I am able to find the other functions in git -- '/odoo/odoo-server/addons/mail/models/mail_activity.py', not sure what I'm missing.