I gave away a key on a forum and wanted to check if it was used yet. Even after many hours (I'm sure the key has already been used by now) it keeps giving me this message. Why does it say it like that instead of simply telling you the code's already been used? There must be a good reason.

Someone entered code to check and didn't click "cancel".

You can check whether it has been redeemed in the 'Orders & Settings'.

It's a known bug.
the code's already been used.

Yep that is correct.

@OP - If you scroll through your list of purchases or do a search, you can see where you have gifted the code to someone and if they have claimed it.

It was a free key.

If one person "checks" a key, the key gets locked out for a period of time with that message (not sure how long). If it got used then the message changes to one saying that, but if the key didn't actually get redeemed then eventually it should work again. Free keys appear there.