Minor OData-Protocol, section 11.2.2 states
Properties that are not available, for example due to permissions, are not returned. In this case, the Core.Permissions annotation, defined in OData-VocCore MUST be returned for the property with a value of None.
In this case refers to the case of missing permissions. Properties that are unavailable for some other reason must also have their omission explained by an annotation, lest the client wrongly assumes null or default values for them.