Translated from NOS TV in the Netherlands:
Rare turtle beached
According to director Marc Damen of Blijdorp Zoo in Rotterdam this is a Kemp’s turtle. That is a rare animal: since 1957 it had been seen five times in the Netherlands. The turtle is native to the Gulf of Mexico. It is threatened in its existence there.
Probably it had sailed in ballast water of a container ship. Damen: “If the ships reach the open sea, then they take in ballast water. And before they, for example, enter the port of Rotterdam, they will drop that ballast water. Probably the turtle had been sucked up.”
See also here.