If you run ShowEdges, set it to All Edges, and in your active display mode you have tangent seam display turned off, then any tangent seam edges do not show up in the ShowEdge display. This to me is a mistake, analysis modes should be independent of display settings and when I ask for ALL EDGES as an analysis tool, I want to see ALL of the edges!
This is present in V7 and the current WIP.