In the example you illustrated, with the holes being seen from a fairly low angle (resulting in no cast shadows falling on the visible portions of those walls), then you are correct - there would be nothing to actually draw. That said, you might still use some trickery, implying the presence of the holes with a partial curve. The focus largely comes down to communicating - identify what it is that you wish to convey to the viewer, and then do what you can to get it across. Even if the result is not 100% realistic (in that nothing would be visible), consider that a purely stark black and white view of an object is also not realistic.

Take a look at this diagram. What I mean by a partial curve is something like the mark labelled "very shallow", next to the three checkmarks.