Find in all open TextEdit documents

The following example AppleScript code when run from Script Editor, or saved as an AppleScript application, will present a search string dialog box for you to type in the search string.

If a match is found, it then creates a new TextEdit document with the name(s) of the document(s) containing the search string. If no match is found, it displays a dialog box with a message.

if running of application "TextEdit" then
    tell application "TextEdit"
        set docCount to count documents
        if docCount is greater than 0 then
            set searchString to ""
            repeat while searchString is ""
                set searchString to my getSearchString()
            end repeat
            set documentNamesList to {}
            repeat with i from 1 to docCount
                if text of document i contains the searchString then
                    copy name of document i to end of documentNamesList
                end if
            end repeat
            if documentNamesList is not {} then
                set AppleScript's text item delimiters to linefeed
                set documentNamesList to documentNamesList as string
                set AppleScript's text item delimiters to ""
                set docText to "The following TextEdit documents contain the search string: " & ¬
                    searchString & linefeed & linefeed & documentNamesList
                make new document with properties {text:docText}
                activate
            else
                display dialog "No documents found containing the search string: " & ¬
                    searchString buttons {"OK"} default button 1 with title "No Match Found"
            end if
        else
            display dialog "There are no open documents to search..." buttons {"OK"} ¬
                default button 1 with title "No Open Documents"
        end if
    end tell
else
    display dialog "TextEdit is not open..." buttons {"OK"} default button 1
end if

on getSearchString()
    return text returned of (display dialog ¬
        "Enter the search string:" default answer ¬
        "" buttons {"Cancel", "OK"} default button 2 ¬
        with title "Search Open TextEdit Documents")
end getSearchString

_{Note: The example AppleScript code is just that and does not contain any additional error handling as may be appropriate. The onus is upon the user to add any error handling as may be appropriate, needed or wanted. Have a look at the try statement and error statement in the AppleScript Language Guide. See also, Working with Errors. Additionally, the use of the delay command may be necessary between events where appropriate, e.g. delay 0.5, with the value of the delay set appropriately.}