Repost--I deleted this yesterday when I discovered the answer and remembered this on meta. Had second thoughts this morning, since this might be useful to someone in the future. I'll let this go for a week and provide my own answer if it's unanswered.
I came across some hack code that I wrote from a few years back that I need rewrite in c#. At the time, I couldn't find property/method that would tell me if a layer was actually visible on the map. As opposed to the ILayer.Visible property which is the potential for a layer to be visible. IMapLayers.IsLayerVisible looked promising but it didn't return the expected values.
My ESRI incident didn't have a related bug and the resolution to that incident was my own hack that I provided.
How are others obtaining this information about a layer's true visibility within a map?
When I said true visibility, I meant that if a layer was visible and it's parent/grandparent/etc are visible too. 'True visibility' was a poor choice of words on my part, because it could be interpreted as a visible layer that has features within the current extent (which is not what I need).