I keep coming back to this CCTV footage which is really spooking me:
https://livetrains.co.uk/jamCams/archive/00001.03901/vid/1614808846.mp4
I have been watching the movements of the driver and passenger (car in the front left) on repeat: it doesn't look at all like normal movements from the driver stopped at a red light, and to me it appears as though the passenger in the back is trying to escape / lean out of the window.
As the WS said earlier, the timings do appear to fit... Absolutely horrible if this is SE trying to escape / get help from the passengers in the adjacent cars. Makes me want to cry at the thought of it.