anti corrosion, I believe. I found it on good/pricey cases only.
Thanks. I searched Google and found a comment from someone on Amazon with the exact same paper inside a CoolerMaster case:
Question: There was a folded sheet of heavy paper in the case labeled "WRAP VCI - Treated with volatile corrosion inhibitors (VCI). Face opposite side to metal. Do not unwrap until ready to use." I've been building cases for 20 years, and I've never come across one of these before. What is it for?
That describes exactly what was in my case. I suppose it is similar to putting a packet of desiccant inside the packaging, but for a different purpose. I've just never come across it before.