Hardwood floor disclored (rotting?) near pipe

Hello,
I own a house that contains a rented apartment on the upper floor. The entire house was centrally heated via an oil furnace and hot water radiators.
A few years ago we had a contractor install electric baseboard heating upstairs so the tenant could control and pay for their own heating. As a result the old radiators were removed and the water pipes leading up to the radiators were cut just above the floor level and capped since they were not required anymore in the upstairs lodging.
It was recently pointed out that the hardwood floor is discoloring (as if rotting) in the area around the capped pipes where the old radiators were. I have not noticed any condensation around the capped pipe and no leak is evident. This is evident in more than one room so it seems related to the capped water pipe. What could be causing this? Could it be condensation from the pipe, although it isn't evident when I inspect it.
Any suggestions welcome.
Basil
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There should be no water in these pipes if the heating is disconnected, or is it the case that you are still using hot water for the downstairs (in which case there could be water pressure in the upstairs pipes). The right thing to have done would be to disconnect those pipes entirely at their source. Having a pipe like that buried in the wall, under pressure, and gradually rusting away, is a prescription for trouble.
I don't think condensation could be an issue since the water in the pipes is either warm or at ambient temperature, but there could be leaks. You say you don't find any leakage, but it sounds like there is some. The black color you are probably observing is due to a chemical reaction between rusting iron and the (probably) oak flooring.

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A moisture test will tell if you if the wood is wet.
M Hamlin

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