Steam says game isn't installed, but i still can play it via shortcut.

[Leozi]

I pre-loaded Nier on thursday, played normally friday and saturday, but when i went to play it today, it wasn't installed on my steam, the button doesn't say "play" but "install".
So i went and checked the steam folder and found the game there, even played it for some time, but it's still "not installed" according to steam. Need some help fixing this.

Thanks in advance