Yep, it's a circlip. Only a careful examination/re-dismantling of the laptop's innards will reveal where it belongs.
Circlips don't usually fall off though, more often than not they're a real ?!!@! to get off! -- very odd.
As others have said, it's a circlip, aka C-clip or E-clip.
In a laptop the most likely use is to retain the shaft on a fan or on an internal optical drive so I'd look there. But the funny thing about these is they're a PITA to get off and sometimes go flying across the room when you manage to get it to snap out of it's groove. So it's possible you and your laptop were in a room where someone else was working on something and their circlip snapped off and flew into your laptop keyboard, then worked it's way inside.