The rings on the photo proved that this was the young sea eagle which had been poisoned last autumn. It recovered in a bird asylum in Zundert. After recovery, it was freed last October in Biesbosch nature reserve. The bird had not been seen since.
Apparently, it flew all the way from the southern Netherlands to the Lauwersmeer in the north. There are now five sea eagles in the Lauwersmeer reserve.
UPDATE: this eagle near Nijmegen in November 2015: here.