VMS Help Lexicals, F$LOCATE, Return Value  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    An integer value representing the offset of the substring
    argument. An offset is the position of a character or a substring
    relative to the beginning of the string. The first character in
    a string is always offset position 0 from the beginning of the
    string (which always begins at the leftmost character).
    If the substring is not found, the F$LOCATE function returns
    an offset of the last character in the character string plus 1.
    (This equals the length of the string.)