Hi folks,
while on Skellige I stumbled upon a turned over wagon about eastish of Fyresdal. During the quest I had to examine the wagon site and a large boulder came down the mountain.
Geralt then went up to see what caused that and saw some nekker footprints in the snow -- following those you get into a cave where the entrance collapses right after you went in.
Exploring the cave you have to fight a lot of endregas and at the end an Arachas (dropped the venom).
Going further on you find a sort of shoe that looks like a nekkars foot (so someone was leading you there).

You end up in an abandoned tower where you find sketches on a wall (looks like a child draw em). On one you see yourself and that boulder you evaded, on the other the endregas finishing you off and on the last a couple of fellas meeting in the woods.

Looking at that picture Geralt says something like that is the meeting point and now its "high time to find those pranksters".

Well.....at this point I am a tad stuck. The quest (whichever it is) doesnt show in my questlog (didnt even show on the earlier stages) and I have no idea where I should go now to finish this.

Anyone who can help me out here?

Not sure where you mean. Don't recall stumbling across it and I did most quests in my first play.

Where abouts is the wagon? Right click to mark the spot on the interactive map and share back the link if you can remember?

http://witcher3map.com/s/#6/-55.522/-11.854/w=-55.937,-14.722 (This is near the Earldorman at Fyrsedal)

SPOILER:
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I believe you are inadvertently in the middle of DLC quest Skellige's Most Wanted, which you pick up in Fyresdal.

Ah, that would be why I totally didn't recognise it... I played through before the DLC was added.

I guess I'll catch it this go around.

Ah right..thank you. Somehow I totally oversaw that one in the log -- can swear I didnt get any messages while trodding through the quest but when loading the game this morning it all was there (finished steps included).

Thanks