Sink stain emanating from overflow hole

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.
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On 8/1/2020 9:17 AM, TimR wrote:

Probably fumes from the trap and drain system. Put some water down it and flush it out and it may help.
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Unlikely to be fumes if the stain is where liquid would run.

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On 8/1/2020 4:07 PM, %% wrote:

WTF do you think is happening in that drain? You wash down all sort of caustic and organic materials and they sit in the trap where that overflow connects. Some washes back in the bottom of it. sits and oxidizes. Then it gives off gasses that rise and coat things.
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Most likely something like emptying the bath is producing a surge up the drain which sees some dirty water from the drain coming out the overflow and its that the stains the sink.

Its less clear that that is happening much with that infrequently used sink.

But gases don’t come out the overflow and move down to the drain.
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On 8/1/2020 5:12 PM, %% wrote:

Key word -- infrequently Dirty water sits and rots.
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But doesn’t move out the overflow down to the drain.
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On Saturday, August 1, 2020 at 7:24:57 PM UTC-4, %% wrote:

The stain is exactly where liquids would flow and shaped like they would be .
But it's hard to see a backup happening out the overflow that didn't also s how at the drain.
I thought of fumes, but they would have to be heavier than air.
I scrubbed it off this morning, too late for a photo. If it recurs I'll do that. This is the sink I use to brush my teeth, I don't think wife or dau ghters ever use it, certainly not on a regular basis. So it gets about one drinking glass of water per day, but that's every day.
On the other hand. About ten years ago I replaced the faucets in that sink . I don't remember why but I wouldn't have done that job without a darned good reason. I do remember the old ones didn't want to come out, I think I beat on them with a rubber hammer for a long time before they moved. I ha d to pull the sink from the vanity into the backyard to get it apart.
So it's not impossible I screwed something up when I replaced it. But it h asn't backed up or dripped underneath, so I can't have done too badly.
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That’s the crucial detail that needs to be explained.

That might well be due to how the overflow comes into the drain before where the plug goes in the drain hole.

And wouldn’t produce a stain that looks like liquid running down. The stain would be much wider and less defined even if the fumes were heavier than air.

Since you never put the plug in and never fill it, I'd block the overflow with something and see the stain never show up again and then block it permanently but so that the block can be removed if you sell the place or start using it as a filled sink again later etc.

faucets in that sink. I don't remember why but > I wouldn't have done that job without a darned good reason.
The usual reason is because you cant stop it dripping and cant work out how to replace the washer seat etc.

Yeah, they can seize quite easily with some poor designs with no way to get enough leverage.

Yeah, maybe so a backup ends up with enough backing up the overflow to be enough to stain the sink with that dirty backup.
< But it hasn't backed up or dripped underneath,

But might have ended up with some sealant etc that you used to stop it dripping redirecting a backup up the overflow accidentally.
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Could you have a lizard or insect that comes out at night?
Fred
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