SCP Foundation

it abducts low-level personnel over several years

I think if even one janitor went missing the Foundation would launch a full-scale investigation for a potential breach. It would get caught pretty quickly.

Also it's still killing for no reason. There's a lot of predators on the site already, I think there needs to be more to it than a monster that eats people and hides well. Maybe it gets declared 'neutralised' and is re-released back into the world, and the article can be about the hunt to re-contain it?

Otherwise the twist will come out of nowhere, which isn't very fun.

Actually, the best twists often do come out of nowhere; they're just set up so that they make logical sense in retrospect. SCP-2770 is a phenomenal example of how to pull this off.

I do like the idea of an anomaly that feigns lesser properties to disguise its true nature, but like rubberchickenzilla pointed out, the Foundation is used to harmless-seeming things turning out to be lethal: If a janitor goes missing, they're probably going to be scrambling the containment teams. What I'd suggest is instead focusing on competent!foundation figuring out that this thing is more than it seems by connecting the dots between many minor, seemingly unimportant incidents like missing office supplies and moved objects.