Yeah, it's one of the most infuriating tasks to complete as the robbers don't ever leave the forest cover, and don't seem to leave tracks on the overland map until AFTER you've zoomed in on their location. You have to manually zoom into each tile, and look around for human tracks. If they're nearby you also won't be able to exit the area to the overmap, which is a good indicator. You will lose time, sanity, and unless you savescum (rollback your save to when you just accepted the quest), your save file size is going to massively inflate from all the local maps you've visited. The reward is pretty good though (100 squirrel pelts worth of goods), but I think I'll just save myself the effort and skip that one, next time.