Pfff
@Sammy *******
. You say you "never heard of this requirement" (notify addres of stay with TM30 !!!) and then you say "maybe because I have always declared their address on TM6?". No....THAT you did with that TM30 form. NOTHING to do with TM6 !!! THAT has been scrapped for now.
You mixup those two things. TM30 = Notification of stay and TM6 is the departure card (In the past imm. at the airport keeps the arrival card part)