I recently encountered an unlocked 378 door as well.
A few months back, I was on probably carriage 4 of the 5-car Class 378, on the South London Line heading to Clapham Junction. There were frequent loud 'bangs' coming from the back of the train. So I walked to carriage 5 to discover that the cab door (the one separating the driver and passengers) was opening and closing from the acceleration and decceleration of the train. So everytime the train started decelerating, the door would shut with a bang. The emergency handle to open that door was untouched, so it was unlikely a random anti-social passenger opened the door.
For a moment, I thought if I should report the unlocked door. It was actually a late night train heading to Clapham Junction. If I reported it, the train would be delayed and I would miss an SWR connection from Clapham. So I took no action and just stood near the door and watched the scenery from the rear of the train (like a crank would). No passengers seemed to care at all, there were busy with their phones, chatting, on headphones, etc.
The train arrived at Clapham without incident. At that point, I thought there was no need to report it. When the driver changed ends, he would discover the problem anyway.
So back to the incident at the beginning of this thread - it may be that the door was unlocked and the youths were there at the right place and right time. There was probably no T-key. It seems they entered the cab from the passenger saloon and then later opened the cab-to-platform door.