When he went (en tōi elthein auton). Luke’s favourite temporal clause = “on the going as to him.”
That (kai). Another common Lukan idiom, kai
=hoti after egeneto, like Hebrew wav.
They (autoi). Emphatic.
Were watching (ēsan paratēroumenoi). Periphrastic imperfect middle. Note force of autoi, middle voice, and para-. They were themselves watching on the side (on the sly), watching insidiously, with evil intent as in Mar 3:2 (active).