In early versions of Pro/E there was the option to create datum planes that were completely internal to features. The datum planes were automatically hidden from view and only were visible during defining or redefining the feature.
If you can's see them, check to see that all layers are visible and that datum planes should be visible.
There is also the chance the feature was created by referencing datum planes in other parts or assemblies; there would be an external reference.
You can also look at Tools/ Program to see a more detailed structure for the part.