Within the past few weeks, a new dark deposit has appeared on the inside of the coolant expansion tank. Coolant is the boringly usual Prestone, not more than 3 years old, and the same as nearly forever except for a brief experience with Dexcool 5 years ago. Schmutzy-type deposits have shown up over the years, but they clean up easily. The new stuff seems pretty well plated on and only comves off with scraping.

No leaks anywhere, thermostat seal is tired but intact. I'm taking the opportunity to flush and refill with fresh coolant, but I'll just throw the question out - anyone seen dark deposits like this before?