Missing character glyphs in fonts have a wide variance of appearances, just like LATIN CAPITAL LETTER A glyphs.
If it will help to understand this issue, here is how it works from a font perspective. The missing glyph is the first glyph in any font. This is mapped to U+0000 and the system correctly substitutes the glyph mapped to U+0000 any time a font being used lacks an outline for a called character. (Unless the application engages in arbitrary font- switching, which is another story...) The font specs say that the missing glyph should look like a hollow square. Actual appearances of glyphs in fonts are always left up to the font developers. Since the glyph is mapped to null (0000) and looks like a box, we often call them null boxes. More detailed information about how the missing glyph is inserted can be found in the font specs for the cmap table: http://www.microsoft.com/typography/otspec/cmap.htm Best regards, James Kass.
