Hello,
"When the rejection is cancelled, REJECTION_STOP event gets triggered."
That's good, you can listen out for the event with a wait-for-event step and send the notification.
"But I cannot see anything in WS000000038 which triggers this event."
It does not matter where or how the event is created. Why would it be in WS000000038?
"Also, when the release is cancelled, RELEASESTEPCREATED event gets triggered"
That is not an event indicating that a release has been cancelled, though it can sometimes occur at the same time one is cancelled. You could listen out for that event and then do some extra checks to see if it really is a cancellation.
regards
Rick