if you are inside a script, try to add a carriage return character to the search string:
"searchstring" & char(13)
(as far as I know, FileMaker still works with the old Macintosh cr character and not with the new unix line feeds (char(10))
That should work.
but in my case it does not seem to work. I tried to add both Char(13) and the "real" carriage return (¶).
(Also tried to add Returns both at the beginning and at the end.)
It seems as I've found a solution that works. I use the FilterValues function for it.
For this, I first created a value list that contains all the values of the field. I call this value list "FieldValues". Then, in my script I have a step that says
IsEmpty ( FilterValues ( ValueListItems ( Get ( FileName ) ; "FieldValues" ) ; "002" ) )
If 002 is not an element of this value list, then the script does certain script steps, if not, then other steps are being processed.