Hi Drew
Just a couple of things that could be checked -
1. The _viewPaneID property holds the ID from the DAML for your pane, correct? It should be "ClosePane_Pane1" from the example snippet from config.daml below.
<panes>
<pane id="ClosePane_Pane1" caption="Pane 1" className="Pane1ViewModel" smallImage="Images\GenericButtonGreen16.png" defaultTab="esri_mapping_homeTab" defaultTool="esri_mapping_navigateTool">
<content className="Pane1View" />
</pane>
</panes>
2. While debugging your add-in when you place a breakpoint on this line below does it get hit? How many panes were open with that the _viewPaneID? When a wrong InstanceID is passed to ClosePane method, I do see that nothing happens (seems to crash silently with no exceptions).
myPaneInstanceIDs.Add(pane.InstanceID);
Thanks
Uma