As has been said, the content of the panel on the stock in question is dead unless the key is on and the panel is active. From my experience with Desiros though an unlocked panel gives someone the use of the PA, as the panel doesn’t arm until something like 3mph when all controls in the panel become active, including the open local door.
This has been causing issues as the panels don’t always secure after use, either because the cord from the handset gets in the way or the latch itself gets in the way.
I was speaking to a guard the other day that said a group of undesirables has an L key and has been interfering with the train, usually only (I say only) playing music or swearing down the PA, some have been actually interfering with the door controls which is obviously dangerous.