I surveyed the place and the memorial plaque is still there. It's a memorial plaque in front of a big oak tree. A recent picture can be found in Wikimedia Commons (see https://commons.wikimedia.org/wiki/File:Amerikaanse_Eik_Heverleebos.jpg) and I added the picture and inscription to the OpenStreetMap node. The real node is a little more to the East though, the main OSM map still shows 2 trees at the moment, I suppose that will change when the image gets updated. I will close this remark.