Thronebreaker: The Witcher Tales

Treasure maps are collected from looting, but mostly by being rewarded with them for completing side missions and speaking to characters that you find throughout the game in the various towns and holds. If you see someone with a speech bubble above their head, speak to them; they may give you a treasure map. There are more than fifteen in the game and you will need them all to find golden chests, so if you're going for the completion, this will unlock as you're attempting to find all of those.