An ability everyone would get. Causes backstabs, steal attempts, sleight of hand attempts, saps, and any stealthy shenanigans against you to have greatly increased chances to fail. In exchange, you lose 10 or so stamina every minute to indicate the mental and physical strain of being so on edge. You can't be in this stance if you're not standing. Entering the stance consumes 10 stamina.
Additionally, being in this stance would have the delays incurred from attempting to subdue or 'kill' a person to be halved.
Would not increase ability to find hidden people, nor the chances of success of your own combat skills.
Would change your ldesc to "appears to be extremely on edge." and would cause you to occasionally emote about glancing around alertly.
Exit the stance automatically if you sit/rest/are subdued/enter combat.
I could see this having a weird interaction with threaten
raider walks in
grebber enters sigma stance
raider threatens grebber
epic mexican standoff
But why though