Character code of unknown character-character, e.g. square or question mark romb

U+FFFD REPLACEMENT CHARACTER = 0xEF 0xBF 0xBD

Unicode has two symbols for unknown characters:

• □ (WHITE SQUARE, U+25A1) - Replaces a missing or unsupported Unicode character.
• � (REPLACEMENT CHARACTER, U+FFFD) - Replaces an invalid or unrecognizable character. Indicates a Unicode error.

Sources

• Quora - What symbol is the square box shown for non-representable Unicode characters?
• FileFormat.Info - Unicode Character 'WHITE SQUARE' (U+25A1)
• Wikipedia - Specials (Unicode block)