Using the API is not the only way a webhook can be deleted.
The following reasons can cause webhooks to be deleted.
Deleting a Box application deletes all webhooks associated with the application.
Deleting all access tokens associated with a webhook.
If webhooks could not be delivered for a system-determined amount of time
then Box automatically removes the webhook.
In all of these cases Box would send a webhook payload with the WEBHOOK.DELETED
event name to the notification URL. In this case the body of the payload will
include the following additional information.