When they had found him on the other side of the sea (other side than that on which the miracle took place, and yet near Capernaum. This contradicts the exposition which would make the site of the feeding to be on the Western side), they said unto him, Rabbi, when earnest thou?
and how happens it that thou art here? The πότε ὦδε γέγονας; is difficult to translate. The when? practically includes the how? also. The difficulty lay in the time. They were sure that Jesus had not started before the disciples, and they knew that there was no method by which the lake itself would have been available, and they want explanation.
The news of his crossing the water after some fashion that would ally him to Moses, Joshua, Elijah, may easily have got disseminated, one report or another being rapidly circulated.