When using Find Usages on an entity, I would expect that all references to that entity are shown.
But this is not the case. When a retrieve action of that entity uses an association, then it won't list in the results. In Mendix documentation this is explicity mentioned. You should use Find usages in actions... and check Retrieve to find these locations also.
This is not intuitive and makes it difficult/cumbersome to find ALL references of an entity.