I´m going crazy with this problem - perhaps you can help. I´ve got a very simple scene, just a mesh for walls (5x5 meters), one for the ceiling and one for the floor. All meshes are about 20 cm thick and the whole room is closed, without any leaks.
I added a directional light, fog and skylight. The baked scene inside the room is completely black without any light leaking - and this is correct. But my moveable character picks up some light from somewhere . I also decreased the number of the detail cell size - this helped a bit, but still some lighting on the character appears.
Using the latest 4.20 version.