Есть спецификация ECMA -376, в которой описаны функции, которые приняты в Open Office XML, для функции find есть такое описание:
FIND ( string-1, string-2 [, start-pos ] )
Description: Performs a case-sensitive search using a lexical comparison for the first occurrence of string-1 in string-2, starting at character position start-pos within string-2.
— string-1 is not found within string-2, #VALUE! is returned.
— start-pos designates a position outside string-2, #VALUE! is returned.