I have had this question come up a few times and thought the explanation was worth a blog post. Let's say you have the following scenario of a wall and column:
So why does the column appear to be transparent in the Detail callout?
The reason for this is that, when unjoined, the wall and column occupy the same physical space. You do not see this display in your Floor Plan and Floor Plan Callout because of the "show family pre-cut in plan views" parameter of the column family.
This parameter determines whether the column displays based on the cut plane specified in the project's view or within the family. Keeping this parameter checked results in columns that always display the same regardless of the project view's settings. More information on this can be found in the Specifying How a Structural Column Displays in Plan View document in the Help menu.
So when this parameter is checked, you are not seeing the 'real' relationship between the elements in your Floor Plan and Plan callout - you are seeing a representation of the column based on the cut plane in the family.
To further clarify, if you edit the column family, go to Family Category and Parameters and clear this checkbox, you'll see the that column displays with the same sort of transparent appearance in all views, not just the detail.
The way Detail Callouts are generated internally is different from true 'plan' views and they do not use this parameter, so they show consistently based on the cut plane of the project regardless of whether it is checked or not.
The ways to approach this would be to join the wall and column where applicable (so their geometry no longer overlaps) or to use a Floor Plan callout when needed instead.
I hope this helps!
The Revit Clinic