Forbes Witcher 3 review

Speaking of question marks, there's this one Undvik on the middle north side of the lake, but nothing's there. Is there a related underground area I just haven't found yet?

Progress with the story.

But I finished the story and it's still there.

Then either you went through a choice that skips that part or it's a bug.
Remember, depending on your choices, some stuff will happen - or just won't.
Or perhaps you were there, but missed to reveal something, for example some power stones are available only during the main story and if you won't explore the area you'll miss them. And they're all represented as ? till you come close to them.

Here is a map of Undvik. Which location do you mean? Cave or treasure?

It's that hidden treasure on the north side of the lake, that's greyed out for you. No great loss most likely. Thanks!

Just three 16 level bandits with the letter giving treasure location. Maybe they have grown bored waiting for you and buggered off?
Chest with some average loot in the lake. No great loss indeed.

FYI PS4 update is out!

I seem to remember killing them. Can't remember if I got a key.

Originally Posted by Ovenall
FYI PS4 update is out!

XBone patch is out too. PC is still on hold but should be out any minute now. Or hour.

Originally Posted by Thrasher
I seem to remember killing them. Can't remember if I got a key.

It was a letter.