Mobs will drop air. it has no texture on the ground, but in hand it looks like the growth lilypad thing. It can be placed as air but it is very wierd.

@Peach774 Can I see a picture of it in-hand? Thanks.

More is happening than just dropping from mobs.

For one, tile.air.name 0 is visible in NEI.

Next, with Reliquary installed, Applied Energistics is unusable. All of the drives are filled with tile.air.name 0. When you put one of the AE drives in a cell workbench, this happens: