When he went (εν τω ελθειν αυτον). Luke's favourite temporal clause = "on the going as to him."

That (κα). Another common Lukan idiom, και=οτ after εγενετο, like Hebrew wav.

They (αυτο). Emphatic.

Were watching (ησαν παρατηρουμενο). Periphrastic imperfect middle. Note force of αυτο, middle voice, and παρα-. They were themselves watching on the side (on the sly), watching insidiously, with evil intent as in Mark 3:2