Non-existant existance in my account?

You do own it.

You made me check your account for nothing...You allready own the game.

But I don't see it anywhere in my account and I definitely don't remember ever purchasing it

That's really messed up... I seriously never played it but whatever. Thanks for helping out guys

I am confused. Maybe you have a personality disorder and you forget what your other part does.

Actually, this happened to me too. A week ago, this game suddenly appeared on my game list on my profile, and it said i played it for 0.3 hours. I don't own the game, nor did I ever installed it or anything. I can't see it in library, but i can click on install on my profile game list, download it and probably play it then. I'm not sure does it work since i haven't downloaded it yet. And yes, i can see the game in library when i start download and there is a play button there. Not sure why it happened, but who cares? It's a free game.

existception

a time traveler going back in time and play Serious Sam on Steam ...

A very long time ago I thought that I got Serious Sam HD The Second Encounter for free because it was on my Played Games List but it turns out when I downloaded/installed it, it was a demo and not the full game... I also played it when it was free on weekend but yeah, this is a steam glitch.

that game also shows under my games list and is not in my profile its because i played the demo of it 9 months ago.
the demo was called the full game at that time and 1 of the dlc was to unlock single player and coop mode

it's because you played a pirated game on fakesteam/greenluma etc. and it appears as serious sam so you can join others and so..