So as it turns out there's a plethora of hats in the game files that don't seem to drop : r/Vermintide
Warhammer Vermintide on Twitter: "Sienna's new Premium Skin and Hat are available in the in-game store on PC 🔥 View all the new Premium Cosmetics here: https://t.co/CD9dLzU40x https://t.co/EulkxNuQiS" / Twitter
Some choices FS made about hats that I don't understand - Vermintide 2 - Feedback - Fatshark Forums
How To Get Cosmetics In Warhammer: Vermintide 2 | Shacknews