If cells in range not in range then...

Sub monitoring()
Dim lrow1 As Long, lrow2 As Long, c As Range, i As Long

lrow1 = Sheets("Data").Range("A" & Rows.Count).End(xlUp).Row

Application.ScreenUpdating = False

With Sheets("Elemzés")
    lrow2 = .Range("A" & Rows.Count).End(xlUp).Row
    For Each c In Sheets("Data").Range("A2:A" & lrow1)
        If IsError(Application.Match(c, .Range("A2:A" & lrow2), 0)) Then
            lrow2 = .Range("A" & Rows.Count).End(xlUp).Row
            c.Offset(0, 1).Value = 0
            .Cells(lrow2 + 1, 1).Value = c
        End If
    Next c
End With

Application.ScreenUpdating = True

End Sub