These approval notifications are not genenated by ITSM notification engine.Hence, you will not find any notification messages configured in SYS:Notification Messages form.
These are approval engine notifications.
CHG:ChangeAPDetailSignature is a three way join form between CHG:InfrastructureChange and AP:Detail and AP:Signature forms.
You can check the configured notifications for approval process on change form via approval administration console.
The approval notifications are genenrated by run time filters for corresponding approval process.
Approval engine uses AR server API's to create filters run time after notifications are configured for approval process.These approval notifications filters start with AP:Notify xxx naming convention.
Also, you can turn on server side filter logs to check these notifications filters.