mercenary warband with halfling scout:
all markers have
M3, because the slowest member in the warband is the halfling. If a model moves more than M3 you would know it can't be the halfling, so you have to prove it and reveal it or you have to move up to M3.
If during the game the halfling uses his bow, he showed himself, you know all modells which are still hiding are humans with M4, so you can move them up to M4.
Same for other stats and items if you want to use them (for example a modell with better toughness or initiative or cool items like elven cloak).
The Mutation would be to strong, but it was my first idea which would fit for them, but you are right, it can be to good.
@wording: i just translatet it with leo... which word would you suggest?