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.