That's called a "light leak". Indirect light has to be calculated in samples. And they get blurred together in the calculation. Sometimes the settings (which get defined by the artist or rendering tehnician/artist, if you have a team) don't meet the requirements of the geometry/light/material setup. But since one frame with proper settings can take up to hours, people tend to ignore little light leaks, beause its more economical for showcasing work in progress.
Technically it's more of a final gathering artifact than a light leak, per se. I'm using an HDRI environment map on an inverted sphere surrounding the model to provide all of the lighting. When used by itself, this method often produces such artifacts, especially near the "terminator" between light and shadow along rounded edges or curved surfaces. Blurring the HDRI image sometimes helps, or adding another light source to "trick" the final render algorithm into smoothing out the artifacts.
Bottom line, though, is that it wasn't worth trying to fix for a WIP image.
