I was just reading that Cadow was the old spelling for Kadow, a town in Pomerania or Mecklenburg? Which would make sense since my side came from that region around 1910, but have traced back as far as 1644 I think, to a Joachim Cadow.

Incidentally I've found another phonetic link of Celtic Origin, Cazdow, but I don't know of any historical connection yet.