Going to ground behind a Aegis defense line
The same rules of true Line Of Sight apply regardless of gone-to-ground status.  We can assume the enemy has radios, some M41 version of IR scanners, etc and knows there's somebody there.

From a practical standpoint, it's just bad game balance/design to allow someone to shoot you without moving if you weren't able to shoot them on your turn.

Warhammer 40K is very much an abstract game played with pieces that look like soldiers.  It does not try to be a simulation.