if I have partly concealed something isn't that a subset of or variation on my having concealed it??

Well, I wouldn't have thought of it as a subset, Sweetie, though now that you've brought it up, I can see it. I would think of it in terms of degrees: partially concealed, mostly concealed; possibly even exact measurements: 50% concealed, or whatever.
Though if you're talking about a sign, exact measurements of concealment are going to be virtually impossible to get--you'd have to measure too many variables to be practical (from all possible points of view, all poss. distances, all poss. heights of the viewer, etc.). So a statement about how well it is concealed is necessarily going to be subjective, barring it being so well-concealed that no one could question it: completely covered by something opaque, for ex. This is one reason I've always been glad I'm not a judge; so many of these things (such as what constitutes pornography, for ex.) must be assessed according to what "a reasonable person" would think, or something equally vague.