Dehumidifier and basement wall

So, a couple of weeks ago I bought a dehumidifier for our unfinished basement - the walls are poured concrete. After a couple of days I noticed a horizontal dark line about two inches wide stretching along one of the walls. It starts in a corner and just ends a few feet along the wall. It is under the level of the soil outside - the ground outside is well graded away from the house, and it hasn't rained much recently. The line does not feel wet to the touch. I assume this has something to do with the humidity in the basement decreasing relative to the moisture in the ground outside/in the walls?
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Is this a new house?

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Art wrote:

It's 12 years old.
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You might try turning off the dehumidfier for a few days to see of the whole wall darkens to match the line. That would be what I would expect.

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