Flamestrike
Legend
Inside the box you cannot see me. If I'm not moving, you cannot see the box move with my motions. If I breathe quietly enough and avoid making any other noise, you cannot hear me. I highly doubt you can smell or taste me. If you cannot notice me with any of your senses, then by definition you cannot sense me.
I did sense you as you were hiding. It was my observation of you (my sensing of you) going into the box that prohibits you from attempting [the hide action] once you're in there.
[I watched you go into the box] = [I watched you go into hiding]. And you cant hide from a creature that is watching you. Nothing you can do inside that box removes from me the memory of watching (sensing) you hiding from me.
Remember, in the real world there is no artifical parsing between [enter box] then [hide]. The entering of the box is part of the act of hiding.
Empirical testing will bear this out every single time.
The reason 'sensing' is important here by the way, is that it informs your knowledge of where the creature is. In this case, 'In the box'.
Example:
Take (for example) me hiding a ring in my kitchen. I place the ring in the second drawer, and close the drawer quietly. A friend (A) closely watches me do so. Another friend (B) is outside the room, with earphones and a blindfold on, behind a closed door.
At no stage is the ring hidden from either myself or A. In game terms we dont need to take the Search action to find it. Our knowledge of where it is (obtained via our senses, watching the ring get placed in its hiding spot) is objectively correct, and as good as us looking at the ring, smelling the ring, or touching the ring inside that drawer.
Now when I invite B into the room, the ring is hidden from him. It got placed in its hiding spot, while he was not observing it being hidden.
I now ask B to find the hidden ring.
In order to find the ring (in game terms) B must either a) take the search action (representing him rifling through the kitchen looking for the ring) vs a DM assigned DC, or b) make a lucky guess.
In addition to these two methods of locating the ring, either A or myself (getting tired of watching B hunt for the hidden ring) could also tell him where the ring is. Such information almost certainly instantly makes the ring no longer hidden relative to B; he no longer needs to take the Search action to find the ring, and no longer needs to make a lucky guess to find it.
B is now on a level footing (relative to the location of the ring) with both myself and A.
Even though the ring is no longer hidden relative to B, bear in mind B still hasnt seen or otherwise sensed the ring. Its just that A and I have now shared our objective knowledge of the rings location and presence (which we obtained via our senses) with B.
Get it yet?