free steam key

He meant that the key has most likely been taken by a bot (a program that searches threads for Steam redeemable keys to send to its creator), or by an ungrateful person who took the key without warning (we call them ninjas, because ninjas are stealthy).

To avoid the first thing happening, you could substitute some characters from the key with symbols, like this:
XG?K4-M!3QH-36I%I ?=C !=T %=Z, so you can at least be sure that the key is taken by a human.

By the way, there's a thread devoted to the dumping of free keys, https://www.steamgifts.com/discussion/mn6hI/orphan-keys-drop-thread, although it's not always on top because we cannot have stickied threads on this forum (yet).

The idea is that bots could (if you don't disguise your key) activate the key for someone (automated).
By the way https://www.steamgifts.com/discussion/mn6hI/orphan-keys-drop-thread Is a great thread for giving away keys like these.