Hi Aya18,
I'm not sure if these scenes would qualify as "hidden" scenes but there are some scenes at only trigger when the MC has certain passive skills that are learned at 15 (exploit, intuit, observe). These scenes are not necessary to progress, (sometimes they will save a character from death, but these deaths are always avoidable by picking other choices.) Also there is a very difficult challenge to pass in Simon's route if you fail the stealth, you can survive Anthony's transformation with max empathy and the discussions about Anthony have variations from the default version.