They didn't seem to when I arrived at T5C a few days ago. The door opening and closing is timed so you definitely can't get from arrivals to departures by running but noone looked in the train. Past accounts on FT seem to imply sometimes staff are watching but mostly they are not, because going from departures to arrivals unchecked is not a security concern.

When departing, each carriage of the train gets a close inspection for people and any left items at T5A before departing passengers can board.