Yeah, very possible, also since having people around really doesn't make cleaning (trip hazards!), maintenance etc easier. Before I started working at the transit authority I once entered an empty station like 40 mins before the start of service, the opening/closing tours really do take a while

My guess is they lock the station up if they need to do repairs or cleaning, even if they don't go around locking/unlocking all 175 stations every night. All the stations do have doors / gates that can be closed / locked.