Nice finding @wvanholder !
Have you tried to play with the serial connection? Just about the CN1 connector on the board. All I have is 5V out and Tx is at 5V. I know Nuki does not support 2 ways audio nor video and I was wondering if this is carrying any useful information.
FYI I don’t own a Nuki but this was useful to me, I am now thinking about using the 5V/GND from the serial connection to power my ESP and integrate it all in the device 
BTW to detect the ringing in 3.3V or 5V I used an octocoupler.
@edobass123 I have the R684 as well, Wim just have V3/M pins connected but that does not matter, you can just follow Wim’s wiring on the top left pins 4 and 3 which are delivering 9V when it’s ringing.