If two figures have already activated this round, well use Luke and Vader for this example. When Vader activated earlier in the round, Luke did not have line of Sight to him (which is one of the Conditions needed to use EOS).

Later in the round Vader gets a free attack from whatever, well use a Elite Officer for this example. Would he be able to use EOS since the conditions of the card was met?

Conditions met being: 'Vader declared an Attack on Luke' & 'When Vader activated earlier in the round, Luke did not have line of Sight to him '

Some say that it shouldn't because its not Vaders activation, but I don't see that being needed as its not 'At the start of your activation'.
Some would say that the activation is referring to whoever the player has Activated right now. So if it was Jabba using Order a Hit and he didn't have Line of Sight to Luke than it would work?
Really need clarification, because two characters might not have LOS to each other, both Activate, then later get a free attack. How does this work?
References if you have them, FAQ isn't helping me here.

It's specified in the game's FAQ, when you use Element of Surprise outside of the attacking figure's activation, you go based on whether the defender had line of sight to the attacker at the start of the current activation. So the question is, "did Luke have line of sight to Vader at the start of the current (officer's) activation?". If yes, then Element does nothing. Also Element always checks line of sight to the attacker, not the separate figure that instigated the attack like an officer or Jabba.

Also welcome to the forums, and just so you know there is a subforum for Rules questions if have any future questions.