What If Dario Is/was the mole in Danys party
So I was thinking Dario would be the perfect person to have spied on Dany for Euron. Its very plausible that they have crossed paths out in the world, being a pirate and a sell-sword. Or maybe he decided to turn on her after she dumped him and he felt scorned. But I bet he knew her initial invasion plans to land at Dragonstone, and from there Euron could have been casing the place waiting for her to arrive. It just seems to me that Dario coming back as the culprit makes a lot of sense.
Edit note: This is purely speculation, I know of no spoiler about this topic. :-)