How are you using this function, in a script or in a calculation field?
If it's in a calculation field, the field must be set to be an unstored calculation.
If you are using the function in a script, then it should always be returning the number of records in your found set. If your find results in a found set of zero records, get ( FoundCount ) will return zero. If your find uses invalid criteria, your found set will remain unchanged and get ( FoundCount ) will reflect that by accurately returning the number of records in your current found set.