When they found him (heurontes auton). Second aorist active participle of heuriskō. Found him after search and in the synagogue as John explains (Joh 6:59) in Capernaum, perhaps that very synagogue built by a centurion (Luk 7:5).
Rabbi (Rabbei). See note on Joh 1:38 for this courteous title.
When camest thou hither? (pote hōde gegonas). Second perfect active indicative of ginomai. “When hast thou come?” We sought you anxiously on the other side of the lake and could not see how you came across (Joh 6:22-24).