As a consummate RPG player and serious explorer, i go from one side of the map to the other and comb everything out in detail. So, by visiting question mark upon question mark, i ended up looting a stash that finishes a quest (dirty gold), before finding the item that starts it (i looted the ending place first and the beginning place of that quest with scrawled notes that start it, second). Quest never got to be displayed in the quest tab.

Did i receive experience for that specific quest or did i loose the XP too, besides it being listed with other quests?

If a game marks a quest as completed, by that very fact you will get XP where appropriate.

I have a tendencie to do the same.

I suffered 2 problems :
- stuck in "dirty funds : read the letters" probably because I wen't to the camp before
- scavenger hunt enhanced gear : if you buy map after you already found some diagrams, the quest may not trigger

If the quest didn't trigger, there's a strong probability you won't get any XP.
I think these are the kind of trouble you have with open worlds not forcing the player to discovers quest in the order intended by the devs.

No... Quest didn't start, wasn't registered and as far as memory serves, upon looting that stash, no notification of completion or XP occurred. I couldn't possibly know beforehand it had been quest related. Damn.

That XP i missed would have levelled me up. I explored all the map and looted all question marks and i didn't hit a level (need less than 50 xp). And with that system that you cannot grind and farm XP, i am stuck to my level and cannot regain the XP i missed. Damn that slow levelling, at least they should have made it work properly (quest register first, ending second).

Which reminds me, though, in 1 something similar can happen, too. For example, in temple quarter (act 3), upon visiting Kalkstein at swamp, if you felt like butchering Salamandras in former Squirrels' camp, the main quest line for Bleimheim ended immediately with all phases skipped and failed (and xp missed). But then again, quest would be registered at least. Now it is not even displayed in quest log, i found about it from guide (yes, thankfully, finally, free guides of the game started popping up!)...

May be it's not a big deal.
I've just finished : Griffin basic gear -> no XP reward.
So may be, even if there is a quest in your journal, this one didn't gave me XP.
No sure if it's the same with Treasure Hunt : enhanced gear.

Sorry, right now i am in velen and maps are chaotic, so i do not remember perfectly. Those 3 treasure hunting quests are near to each other, though. Two of them end in the old house by the old mill. The second needs a key looted from a corpse in a nearby bridge over water with drowners and patroller on horse.

The three question marks near the mill is where the documents which start those missions, and the vaults that upon looting, complete them. If you screw up the order, though, like me, you will probably miss a quest entry. I thought that the absence of XP reward in my case, though, was tied somehow to the missing of that particular quest's entry into the log...

So the cheats in the old house by the mill, you go under the house and there is a locked chest down there. The chest has a key into but won't let me open it. Just says. Read letter that I picked it. On the chest it doesn't say locked says unlocked and I can see the key but still won't let me open it. I think it's a bug in the game. Thanks for the info.

I believe all the treasure hunt quests are for the Witcher school gear.

If it's one of those quests I don't believe you missed anything by not getting the quest. I got them all and I don't recall a single one giving me XP, I believe they are solely there to help you locate the items and nothing more.

EDIT- Ignore this, was being stupid.

Then you believe wrong.

Many treasure hunt quests start by reading a note/letter/journal/etc looted from a corpse at a ? marker location, such as guarded treasure, bandit camp, etc. It's usually a short little search for a nearby chest, but sometimes they can send you far afield(Family Fortune in Skellige is a great example of this).

Dirty Gold, the one that's most commonly bugged out, is one such quest, it goes in treasure hunts, and you start it by reading a journal looted from the corpse of a spy at one ?, and leads you to another ?. If you loot the treasure before reading the spy's notes, the quest breaks.

Yep, you're right. I usually complete those ones right away so I was only thinking of the Witcher gear quests as I only ever really saw those in my quest log.