Ok, so this situation came up, I had a ninja in Hidden Deployment, where she would have LOF towards a lot of enemy models, as they activate, is it obligatory to delay my AROs, take them at once, or is it possible for the hidden deployment trooper to voluntarily and privately forfeit their AROs? This is something I haven't found in the rules, as the ARO rules explicitly say that troopers MUST take their ARO as soon as they are eligible to do so whenever one of the three following bullets is true. The Trooper has LOF to an enemy model during it's activation. An enemy model Activates within his ZOC. It has a Special Skill or piece of Equipment allowing reaction to enemy actions without LoF
If you do not want to reveal the Hidden Deployment model, you don't need to declare anything beyond "I have no AROs." If you want to DELAY your ARO (vs a marker, or using Sixth Sense) then you need to place the TO marker on the board and declare that this marker is delaying it's ARO.
Reviewing the Rules for AROs I can totally see where you´re coming from, and it would for me seem like that's the case, however... Is there anywhere in the rules where it is specified if AROs are obligatory or not? Does it simply stay implied?
Under ARO: A trooper owned by the Reactive Player can declare an ARO if any of the following is true: Bold for emphasis. You are never compelled to declare an ARO, the "must" later in the rules is specifically in relation to the timing of your declaration and in no way implies that the ARO itself is compulsory.
Do also note that you can only delay with your HD trooper if you would normally be able to delay your ARO (i.e. vs a marker). I only mention this because when I was starting out, I (erroneously) thought that my HD trooper could delay as part of being in HD.