On 8/1/2020 9:17 AM, TimR wrote:
This isn't a problem I need to fix, just a curiosity I wonder about.

We have a sink that is used lightly. It never has the stopper in, it is never filled, there is never a reason for it to have water enter the overflow. The overflow hole is on the near side, the faucets on the far side.

And yet, it has developed a tan stain obviously starting at the overflow and extending down towards the drain.

I don't see any sign water has ever come out of the overflow, nor do I see why that would happen, but there has to be some reason for this.

Probably fumes from the trap and drain system. Put some water down it
and flush it out and it may help.