Sheepstealer or Cannibal cannot secretly alive. The glass candle in the citadel only started burning when Dany's Dragons hatched. The pyromancers started producing more after Dany's Dragons hatched. 

If one / two dragons were still fucking around somewhere on Skagos, their magic would be felt, and the glass candles would be burning before Dany's eggs hatched. 

RIP Sheepstealer and Cannibal