They get such a call from the police chief eventually during the week describing in detail the intentions of Nadia’s ex-boyfriend. It started to turn out that he profoundly came to regret is not actively engaged in his life with Lukas. But he made the decision on some other strategy because Nadia as well as Her parents had ensured communication was unfeasible.
The Real Reason
He tried to prove to Lukas that Nadia was an irresponsible mother by attempting to make it seem that he was held captive while Nadia was super drunk in the picnic area. He assumed that if after a few days, he had just freed Lukas, no one would recognize that it had been him.