Is it possible that subtle caustics calculations are happening even when it's turned off (reflective caustics)
See attached.
Sunlight (HDRI in Corona Bitmap) pointing at a pool, the fireflies are where I'd expect reflective caustics to be if they were turned on. The only issue is, caustics are turned off in both the render setup and the water material. The 'clay' material has reflection set to 0. The pool has a water shader on it so is both reflective and transparent.
If I put a basic material on the water geometry, those fireflies don't appear. If I turn the reflection down to 0 in the water material but leave refraction at 1, they also disappear. Which is making me thing it's a caustic thing?