Statting accessibility "except when watched" and mechanics thereof?

[Extrarius]

In many different horror settings, creatures often seem to have many times their normal power the second all eyes are off of them. For example, vampires 'wooshing away' either moving many times faster than normal or using some kind of warp, or a strong monster can only bang on blast doors until nobody is observing, then suddenly it can break through in a few hits.

In order to stat this kind of accessibility to a reasonable modifier, I feel like first we need to define when exactly it applies.
Some examples of being watched:

• Engaged in combat with somebody, such that they're closely monitoring your movements both for defensive and offensive reasons. (Unless they're restricted to 'Do Nothing' maneuvers)
• Somebody using Aim or Evaluate against you, even if you're not fighting.
• Somebody using Observation, Body Language, or other skills on you that require close attention.

There should also be ways to "break out" of being watched. If you have a single watcher, something like throwing sand in their face might give you a second or two unobserved. Having some kind of Innate Attack(Wall), Obscure, Mental Illusion, or equivalent mundane countermeasures (running around a corner, a smoke grenade, etc) seems like they should allow breaking the "Watched" status at least in some circumstances.

How would you rule this kind of accessibility, and how would you price it so that players might see it as a different flavor of "Emergencies Only" that easily applies when working alone (or with allies, who would learn not to "Watch" to person with such abilities) but not in the company of opponents, especially not when opponents know of your limitations?