I did hear it in the background in a few episodes but haven't been paying full attention ... however in Series 2 Episode 3 at 29:45 it clearly read "CHILD"!! which is where I am up to ... I will be paying more attention from now on as I continue to watch!!
During the intro of the next episode Series 2 Episode 4 at 5:45 the Morse Code read "ZNAKI" then shortly after in a different tone "CHILD"!!