What ARO should declare a decoy (Ambush camouflage) against enemy unit? The same question for mine (in CAMO state or not, enemy outside trigger area).
"I declare no ARO", I think is a good enough answer, considering how camos work: you can do nothing and let them think it's a mine out of range or you just want to save that model for later, etc...
Holoechoes can only declare Idle: "However, the holographic decoys cannot interact with scenery, considering any Activate or Alert declaration as an Idle." http://infinitythewiki.com/en/Holoecho But I can't find similar information regarding mines or decoys. You must declare Idle, or delay ARO, or shoot, etc, if it is CAMO trooper, but not "no ARO", right? I'm just trying to figure out whether a mine or a decoy can declare Idle. If they can only declare no ARO then it is easy to understand where the mine or the decoy, and where the real trooper. In this case, the effectiveness of mines and especially decoys is somewhat reduced.
Off-topic, but I don't think echoes can declare anything on their own. I only see that they can parrot what real trooper declares, and this clause is there to limit certain (actual, not just declared) actions to real trooper only. Other than things specifically mentioned in corresponding rule descriptions, those markers don't do things troopers normally do, including declaring AROs and (I presume) using modifiers that would be in place if they were real troopers (such as -3 for camo in case of fake ambush, or -6 for ODD in case of Partoclus' fake holos).
To expand further, Idle is not a valid skill for ARO, it's listed as a Short Movement Skill which means it can only be declared in the Active Turn. http://infinitythewiki.com/en/Idle