The rules say that in general powers that normally use endurance are visible to three senses. The question is just how visible is that. My character (a mentalist) has a force field and flight as powers. I interpret the rule that powers are visible to mean that in normal situations if someone can perceive my character that they would be able to see perhaps a fringe around my character that indicates the presence of a forcefield and some other hazy affect that indicates I am flying (as if the fact that I'm 10 m up wasn't enough of a clue), but, that these special affects neither enhance or detract from the ability of others to perceive me.
So lets look at a specific example I am hovering in a night sky some twenty meters above a combat doing nothing to attract attention. So I would think I'm pretty hard to see (darkness, range etc.), but, I am being told that anyone within sight gets a very substantial bonus to perceive me simply because I am using powers.
Consider another example. A character has shrinking which gives a modifier to perception based on how much shrinking you have, However, if using a power makes one more visible than normal - the shrunken character is actually easier to perceive than at full size.