There are many cases in ShapeSheet expressions where it's necessary to use the Sheet.123 id to refer to another shape.
However, so far as I'm aware, the Sheet Id of the currently selected shape is not visible anywhere except for the (Visio 2010)
Developer tab > Shape Name dialog,
which is modal and thoroughly inconvenient to invoke every time it's needed.
Here are places where Sheet Id should or could appear:
1. In the title bar of the shape sheet window. Currently the sheet-id-based name is replaced by the shape Name if one has been set.
2. In the Size and Position panel. Maybe only when in Developer mode, if there's a concern that this clutters the panel.
Also regarding Size and Position: The shape name and sheet id should have their own slots so they are easy to read and don't get truncated. Alternatively, leave the shape name in the title bar, but put it first, with "Size and Position" moved to the right, following the same principle as Windows application titlebars.
3. In the Drawing Explorer, maybe in brackets after the shape Name.
-- Graham