Subspace flux is a detectable phenomenon which can be created as a byproduct of an object emerging from subspace into normal space, or which can exist naturally inside the phenomenon known as "chaotic space."
The crew of the USS Voyager detected subspace flux in 2375, upon encountering the "Think Tank," whose vessel was usually hidden in subspace. (VOY: "Think Tank")
Later that year, the Voyager crew detected subspace flux, among a number of other phenomena including graviton waves, navigational distortions, and spatial sinkholes, upon becoming trapped in a region of chaotic space. (VOY: "The Fight")