These stairs are wet.
Originally, there was a leaky water heater and I thought that was the source. Fixed the water heater but stairs are still wet. Then suspected leaky toilet wax ring from upper floor but as I peeled away all the wood surrounding the stairs, they are STILL wet and nothing above below or beside them is wet.
I figured it out. Any of you want to play along at home? Put your guesses in the comments.
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Your wife is messing with your head by sneaking down at night with a pitcher of water?
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No. But thanks for that original answer.
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Walls are sweating and running across the stairs
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The walls are bone dry to the touch. There is no evidence anywhere in the basement of condensation.
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Condensation. Need a thermal break between the wall and the stairs.
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Almost. A good guess. You noticed clearly that the fully insulated basement has no insulation between the stair wood and the wall. In this case the cinder block wall is acting as a transport layer for water from outside. The normal heavy paint that served to waterproof the basement has worn through where the stairs have lightly rubbed on it for 30+ years of use.
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The project for the future includes moving the stairs so this isn’t going to be a problem.
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