First of all, no leak : this allows the light to bounce and create the ambient maps used by almost all surfaces.
Then, the ambient parameter, of course, that should not be pitch black. It's a cheap and efficient way to account to that light so much scattered bouncing it would take no sense. I'ts in the env_light entity settings.
And finally, smart light placement. There will be pitch black zones (darkers ones) but with lights with a dull color (yet powerful value) you can light up those areas without any discernable light source.
And of course, adding visible light where they should be. (and it adds details, too, cool, isn't it ?)