Long ago I tried to implement this mechanic (https://github.com/CleverRaven/Cataclysm-DDA/pull/20160). My error was that I did it alongside with other enhancements and bugfixes, while filthy camouflage idea seems to be rather radical game mechanics change. So later I made a new PR with only bugfixes and never returned to this mechanic.
Copypaste from the PR: Sneaky or pacifist characters can use filthy gear to trick monsters in ZOMBIE monster group. Wearing filthy clothes makes them to “semi-ignore” the player character. While zombies are far away from player character (but still in their field of view), they actually ignore him, but on small distances zombies will still be acting hostile.
Tested this on several types of zombies. They do actually pursuit player on purpose if they are close enough. That’s why “semi-ignore”.
Tested with debug traits. “Semi-ignore” turns into full ignore with no smell trait. Sound is irrelevant here, because player can stand still.
I didn’t do further with this concept, but it could act as a base for anyone willing to continue.