I just looked at mine really close it almost looks like a flaw on the glass on the opposite side of the rear element, like a microscopic fracture in the glass or something. You can only see it it at a certain angle, and if you shine light on the glass or through it, it disappears.
Are you positive it's not a reflection you are seeing? I thought something was on mine before and then realised it was an obscure reflection due to the shape of the rear element. No offence intended











