Thanks for the input, made me think a little... Unfortunately, OneDrive has its own "documents" folder that it syncs. It does not sync the "user's" documents folder by default, unless you set that folder to be the one inside OneDrive. I have just checked this on Microsoft webpages.

And to double check, I took a look at my actual Documents folder, and it's completely empty
completely agree with you here

When I deinstall a GoG game, I let the savegames stay on my HD, so I won`t have to start all over again.

I was talking about the time tracking and all that, I fairly regularly back-up my saves.
Especially when I do a re-install :P

I only know it from my wife's computer where OneDrive (pre-installed with Windows 8 - I have 7) kept trying to "cloud save" everything in the user folder. But if you say it's empty... When you start the game, are any settings "not default"?

This is like literally not possible, for one when you install Witcher 3 without Galaxy your not logged into your GOG account inside the game so the game should have no way of knowing who you are on GOG without Galaxy.

Only thing I can think of is your account was stolen and somebody else downloaded the Witicher 3 and played with your account on Galaxy and your seeing there progress and achievements instead of your own...

Hmm, when I uninstalled/reinstalled System Shock EE (kept crashing now fixed thanks to support *kudos!*) through Galaxy, it kept my "1 minute" play time.

I am not saying that he got it because he played it via galaxy.
May be TW3 saves it online and it appears only in galaxy.

The game is not even installed yet (I am installing it now, it's at 16%) - the info about my play time and achievements are displayed inside Galaxy, under the "Library" tab...

I can't see any reasonable way this info should be there, as you said there is no rule also in my firewall for TW3, and unless quantum teleportation is already a reality, I can't see a way my HDs have been sharing files XD

THIS might be a good idea!
I have been playing the game for one afternoon at my friend's place (wanted to show him how cool The Witcher 3 is!), and it might be that we installed it using Galaxy!
I need to double-check this as it's been quite a lot of time ago, but if this is true then I concur it might be the reason.

Ah it does indeed do that now, it didn't do that a couple of weeks ago so I guess one of the updates did something.
It still tells me I haven't played the game though, which I guess means I haven't played it after re-installing it a minute ago.
Which is kind of dumb, but you know.. GOG.

Wait a minute.. does this mean.. ILLEGAL DATA MINING?!

Much smoke about nothing, like we say in Germany!

Or just ask these people...
And btw. +1 to this answer. It's so obvious in retrospect. Good thinking, mate!

Damn, I always have problems with "these" and "those"!

Yes, I know... I apologise :/
It was a very peculiar thing in the beginning, and I totally forgot about that day at my friend's place.