In Act I, a man named Mathias Tumm appears in the yard in front of Bernard Loredo's residence, next to the stall containing armor and weapons racks, and a large chest. I believe he first appears after:
Geralt kills the Kayran.
He acts as if Geralt has done a service for him, but I do not remember such. He gives you, at least, a recipe for Red Haze. I have confirmed this.
Why does he do this? Does Geralt meet him at any point earlier in the story?