I have an interesting problem that I've never encountered before in all my years in the trades.
I live in a manufactured home (don't get me started! ). In the master bathroom there are 8 to 10 spots in the walls and ceiling that look like rust pitting through the paint, much like you'd see at corners with metal corner bead. Here's the kicker, it starts as a pin hole with the "rust" developing around the hole, then it sort of migrates away from the hole sort of like what a termite tube looks like. These are areas with no metal corner bead, they're random in nature, though concentrated over the corner shower and around one window.
I will try to take some pics soon to share, maybe that will help. These are the strangest thing. I suspect that it may be ants, but I've never seen an ant burrow through drywall, much less multiple places. The affected area is the ceiling over the 3/3 shower and around a window that is 16x20. Southwest corner of the house, for what it's worth.
I really don't want to do any exploratory surgery to figure this one out, but that is probably the only way to know what this weirdness is.