I still can't phantom myself on what happened. The GPS track shows I was extremely close, possibly 20 meters, on my first attempt. After loosing so much time in a zig-zag search, went back to the big huge boulder uphill and re-started from there, and voila', found it right away without problem. It was a little dark due to tree cover so maybe I just didn't see the flag.
Looking at this video
, it was definitely less visible, due to foliage and a oblique tree log