I'm pretty sure this is a bug, I wasn't allowed to place a custom marker on it but a quick search reveals I'm not the only one that has had it. I had just completed the game and the first expansion, removed all the question marks from all the maps and sat back to admire my achievement, but what was this?!?! A yellow exclamation mark? I've never seen one there before!

An area just west of crows perch was highlighted, it was a small pond surrounded by nekkers and not much else, I tried visiting it and different times but it didn't change. I've enclosed some images, has anyone else found this?

This is a UI bug, much in the same vein as the infamous 'merchant' icon that often appears on Ard Skellig where Yennefer uses the mask of Ouroborus (the Ciri destruction area). Best to just ignore it.