Range find

Sub test()
Dim Ws As Worksheet
For Each Ws In ActiveWorkbook.Worksheets
    With Sheets("Dossiers Actifs")
        If Not (.Range(.[A4], .Cells(Rows.Count, "A").End(xlUp)).Find(Ws.[C3]) Is Nothing) Then _
                                          .Cells(Rows.Count, "A").End(xlUp)(2) = Ws.[C3]
    End With
Next
End Sub

Private Sub Worksheet_Activate()
Dim Ws As Worksheet
For Each Ws In ActiveWorkbook.Worksheets
    With Sheets("Dossiers Actifs")
        If Application.WorksheetFunction.CountIf(.[A:A], Ws.[C3]) = 0 Then
           Range("A65536").End(xlUp).Offset(1, 0) = Ws.[C3]
        End If
    End With
Next
End Sub