Using Arista .EDU ultra glossy RC paper, I have had brow spots turn up on finished prints. The spots are not visible when making the prints but reveal themselves after the print is dry. I have had some of them visible only hours after drying, and some of them not show up for many months. This has only affected about 1% of my prints. The spots are fuzzy, brown, translucent, typically small, and typically near the edges.

This only has happened to a small percentage of my prints so it's probably something I'm doing rarely or randomly. To help narrow the cause down, what factors could be causing this? Could it be caused by developer being too old? Could it be caused by developer being carried over to the fixer? Could it be caused by old fixer or by insufficient washing?