Zio_the_dark
The dark one :)
From the DMG:The Bastion and its Simulacrums have an Unbeatiable Stealth check. 36 ~ 46 + 1d4, magical guidance, advantage.. You roll once and remain hidden until revealed.
The DM decides when circumstances are appropriate for hiding. When you try to hide, make a Dexterity (Stealth) check.Until you are discovered or you stop hiding, that check's total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence. You can't hide from a creature that can see you clearly, and you give away your position if you make noise, such as
shouting a warning or knocking over a vase. An invisible creature can always try to hide. Signs of its passage might still be noticed, and it does have to stay quiet. In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach
a creature, it usually sees you. However, under certain circumstances, the DM might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack roll before you are seen.
Hiding is DM fiat as you say.