In the final episodes of Season 2 of The Last Kingdom, the Brothers Erik and Sigefried take London, then abandon it once the forces of King Alfred arrive to try and take it back.
While Uthred and his men are assaulting an empty London, the Brothers have attacked their camp some miles away, killing virtually everyone and escaping with King Alfred's daughter Aethelflaed, the wife of Lord Aethelred of Mercia.
It's clear later on that this is entirely Erik's plan (Sigefried says so) as the ransom for the King's daughter will be hugely more profitable and easier to deal with than trying to hold London.
My question is: How did Erik know that Aethelred would bring his wife with him? It was clearly an unusual thing (Uthred later points out he left his wife safely at home, and King Alfred almost explodes with rage when he hears that his daughter was taken along and has been kidnapped).
It seems unlikely that the Brothers plan was to assault the camp and hope that someone of value was there - therefore they must have known she would be there.
Was there some spy in Aethelred's men that reported back to the Brothers? If so, has such a spy been hinted at or shown in any way in the show itself? Did someone push Aethelred into taking his wife along with malicious intent of her being kidnapped?