Is there a "glyph not found" character?

From the Unicode Spec:

• http://unicode.org/charts/PDF/U25A0.pdf

U+25A1 □ WHITE SQUARE

• may be used to represent a missing ideograph

• → U+20DE $⃞ combining enclosing square

No, there is no “glyph not found” character. Different programs use different graphic presentations. An empty narrow rectangle is a common rendering, but not the only one. It could also be a rectangle with a question mark in it or with the code number of the character, in hexadecimal, in it.

So it is better to e.g. display a small image of the character along with the character itself, so that the reader can compare them.

The glyph-not-found character is specified by the font engine and by the font; there is no fixed character for it.