Currently they are silently ignored. The question is what would be the correct level or style of reporting them:
* only in y2log
* show them in UI as a warning
* other options ...
I just feel neither of these are the way to go.

The naming patterns will be covered in documentation in depth, any users/customers must follow the patterns and test their hooks extensively. 
We should probably make it easy and trivial to test the hook files, e.g. via som yast client that would offer a list of all available hooks along with input boxes for
* custom dir for loading test hooks
* hook names to choose to test

Such a test client could have options for either finding the hook files or executing them if required (or both). If the hooks feature is going to be extensively used by customers, maybe it would be worth helping them with the hook deployment.