I've not tested this at all, but if I remember correctly, it is the event: ModalEvents.hidden - as defined here: https://github.com/moodle/moodle/blob/master/lib/amd/src/modal_events.js#L28
(Without being able to see your code, I can only guess that you'll alias the import as "ModalEvents")
(Without being able to see your code, I can only guess that you'll alias the import as "ModalEvents")