Actually, the guest never mentioned the name “fake code”.
I took them in because they arrived in the middle of the night and furthermore I could also see that a payout was going to be made to my account despite their cancellation.
All this changed in the morning, when I received the mail from Airbnb, saying that the booking was cancelled ( a second time, but this time by Airbnb). The payout on my dashboard was then eliminated.
But yes, I have learned a lesson, that´s for sure.