Rulings from Alex Werner, FFG Game Rules Specialist via the official rules question form:
Question:
Let’s say that Trish Scarborough is at a location with another investigator and there is an enemy engaged with that other investigator. If Trish discovers a clue at the location and uses her special ability, can she “automatically evade” the enemy engaged with the other investigator? If so, what exactly happens - is the engagement broken and the enemy exhausted?
Answer:
Yes, Trish Scarborough can use her ability to “automatically evade” an enemy engaged with another investigator. Once she does so, that enemy will exhaust and break its engagement with the other investigator.
Question:
Let’s say that Trish Scarborough uses Damning Testimony to discover a clue at the location of an enemy and then uses her special ability. Can she “automatically evade” the enemy even if it is not engaged with any investigator? If so, is there any effect other than exhausting the enemy?
Answer:
Yes; it’s possible to automatically evade an enemy that isn’t at your location. That enemy will exhaust, even if it’s not engaged with another investigator. And while nothing additional will happen with Trish and the Damning Testimony, Trish could trigger an effect that required her to evade an enemy, like Pickpocketing. (Note, she can only trigger effects that don’t require a “successful” evade).