When I run the script below I am getting random results.
It says nothing. Describe results you get.
Regardless of above you have to remember about "escaping" wild_characters
app.findGrepPreferences.findWhat = "\\[\\u\\]";
Modify script as I wrote above.
Otherwise your find formula is just "[u]" ==> so InDisign found is --> Direct Manufacturer Name
Perfect, thanks for your time.