Secret aspects

[ZMiles]

As a GM, sometimes I want my NPCs to invoke certain aspects that the players haven't assessed yet. Examples might include if an NPC has a Demonic Co-Pilot that the PCs haven't figured out yet, or if one of them is secretly a psychopath, or something. Normally, invoking implies that you declare which aspect you're using. Is it okay in some circumstances to say "My NPC is invoking his Trouble aspect" without saying what it is?

[Tedronai]

Sure.  But that's not very good descriptive language.
Psychopathy, for instance, should definitely have some outwardly noticeable, though not necessarily immediately identifying, signs, or symptoms.  Some descriptive language having to do with an emotional state that would otherwise be incongruous with the activities the character is engaged in would probably be appropriate.
For an aspect related to Demonic Co-pilot, you might describe it as the character taking on an inhuman, or somehow monstrous, or unnerving visage or expression (without their physical appearance actually changing in the least) as the demon steps forward to take control for a moment

[sinker]

Short answer: Yes.

However you should describe how it is affecting the situation or what the NPC seems to be doing. When you invoke something you're calling on that bit of yourself, which should bring it to the foreground. You don't have to tell the players what the aspect is, but they should get a clue as to maybe what it is. An example that I have used before is the aspect "Military Training." If you're calling on that you don't have to tell everyone what that aspect is, however it should be clear in how you are completing the task that the character has been through some sort of structured and regemented training and he is maybe completing the task on auto-pilot. If you describe that to the players then they will begin to look at that more closely, and maybe they'll start guessing or assessing aspects.

[WillH]

No. Aspects are about narrative importance. They are not just a description of the aspected thing is. If an aspect is relevant to a situation, but the audience, i.e. the players, is not aware of the aspect's relevance, then it has no narrative importance and should not generate any mechanical effect.

[UmbraLux]

As a GM, sometimes I want my NPCs to invoke certain aspects that the players haven't assessed yet. Examples might include if an NPC has a Demonic Co-Pilot that the PCs haven't figured out yet, or if one of them is secretly a psychopath, or something. Normally, invoking implies that you declare which aspect you're using. Is it okay in some circumstances to say "My NPC is invoking his Trouble aspect" without saying what it is?

Yes, allow any relevant aspect to be used even if you haven't written it down.

"Pissed Off" may work to compel that unrestrained demonic copilot for example.  Basically, let the players use what they pick up from your narrative.  It may have a distant relationship to the true aspect at first but, as you add details, the aspects they pick up on will get closer to the ones you chose for the character.

[Haru]

Using aspects your players do not know about is absolutely in your right as a GM. You should however give them the opportunity to discover them. Not by saying "he is invoking his trouble aspect", but by saying "he is spending a FATE point on this action", and then describe, how that changes the action.

Figuring out your enemy's secrets is always part of the solution to defeating him, and using secret aspects is a great way to get the players started on assessing some of them. The more aspects they can figure out, the more they can compel in the final battle.