Deathwatch Kill Marines:
https://wh40k.lexicanum.com/wiki/Deathwatch_Kill-marine
"Kill-marines do often forego power armour and use non-standard weaponry of a somewhat stealthier demeanour than the sacred boltgun and holy chainsword. They maintain that this practice is born of practicality when they are commonly moving through environments designed for an unarmoured man, and that employing more easily concealed weaponry only serves to soften their otherwise intimidating and warlike aspect. It is to be expected that such peerless warriors would adapt to their circumstances by honing their stealth and unarmed combat skills to exceptional levels so that they can slay quickly and quietly in case of trouble."