The editors of the Jewish Monumnt (Joods Monument) have made an investigation regarding the place of death of Barend Koekoek. It appeared to have been Hailfingen Tailfingen, the airfield nearby the concentrationcamp Natzweiler-Struthof. The remains of the victims who have died because of hardships, have been brought to the Crematorium "Unter den Linden" in Reutlingen and buried together in a massgrave. Also the ashes of Barend Koekoek.
The 2nd photo from below at the website of the memorial with names, where is to read "Koekkoek 1910-1944", however without his first name and wrongly spelled, but still recognizable that it is Barend Koekoek. This picture is also placed in the photo carrousel at the personal page of Barend Koekoek
Source: https://de.wikipedia.org/wiki/KZ-Au%C3%9Fenlager_Hailfingen-Tailfingen and http://www.kz-gedenkstaette-hailfingen-tailfingen.de/php/kzht.ar.ve.reu.php