First of all, Happy new year everyone! 
While writing an HdSingleInputFilteringSceneIndex plugin I’ve noticed that I don’t get any _PrimsAdded call when loading a file with UsdStage::Open(...), in my simple viewer, and I only get GetPrim and GetChildPrimPaths calls.
I do get it thou when I add a new prim, while the viewer is running.
Also, I get the _PrimsDirtied for the camera, when moving it around.
I’m using UsdImagingGLEngine in the viewer.
But if I run my plugin in usdview, I get _PrimsAdded and all others _Prims* calls.
I assume this is based on how I’m loading the stage, and before I get deeper in the source code, I wanted to check if there is anything obvious I should look for.
Cheers!