FileMaker is basically doing a "begins with" search for every word in the field.
So if you search for Male, it will find all records where a word either equals Male, or starts with Male, but not records where male is in the middle of a word.
By searching for "No" you are finding the "Not Found" records because they start with "No"
Hope that helps,
Furthering Chad's point,
If you change the Find request using = (Match whole word) or == (Match entire field), you can distinguish between No and Not Found as well