If an invisible creature or object intersects with the effect of e.g. Fog Cloud, is the intersection volume filled with fog or with nothingness?
The use case is to try to spot the nothingness which should be quite easy if the creature or object is either very large or close to the creature searching for it.
Quoting from the srd:
[...] certain other conditions can render the recipient detectable (such as stepping in a puddle).
Avoiding to use physics laws etc etc (keep in mind it's a role playing game) that line of text can help us in that specific situation. Like an invisible creature creates rings on a puddle, in the same way, if the creature (or object) interacts with gas (like fog cloud) ofcourse it can be noticed by observers. Keep in mind, anyway, that in D&D 3.5 concealment NEVER stack and fog cloud (like invisibility) gives you concealment (20% if adjacent or 50%, like invisibility itself, if not) and I want to remember you that:
so I think it will be really hard to "spot" nothingness if in an area with concealment or total concealment since your vision will be already hampered by the fog or whatever.
So there are a couple of interpretations for what 'invisibility' could be.
One would be that it acts like a video camera, capturing the image on one side of an object or creature, and projecting what it captures onto the opposite surface. This would create a human shaped distortion, but would not create a void. I've seen this interpretation only twice before (though that doesn't make it any less viable).
The more common one is that light passes through the invisible object or person as if it weren't there. In that case, any textures in the air would not follow through the space the invisible entity occupies. This is consistent with the idea of carrying around a sack of flour to reveal invisible opponents when they appear. Since the flour lands on the surface of the person, and leaves a void, it creates a profile of the entity in question.
Personally, I'd roll with the latter explanation - though I am quite biased. I once had a player use a fog cloud to detect a number of assassins equipped with potions of invisibility, and, in admiration of that creativity, that is the interpretation I've been using since.
