Assuming there’s treason in Dany’s camp (and I’m very skeptical; I think Euron is just that good at tracking), my candidate would be Olena. If Dany is going to suspect anyone of being a rat, it’s going to be Varys (even without hearing the confrontation between the two, almost anyone could pick up on that). Plus I’m not convinced Olena has any love for Danaerys anyway, given the Targaryen rep and Danys father.
The closest parallel for Olena here would be Tessio in The Godfather. Out of all the Corleone underbosses, Clemenza was going to be the most likely to betray the family once Don Vito died, so Tessio thought he could outwit Michael in order to preserve himself and his family. Olena’s doing more or less the same thing here, even if her motive is now single-minded revenge against the Lannisters