How is water vapor getting into this process plumbing?

Where I work we have chemicals running through pipes. One particular chemical needs to have low water content, so we analyze the source keg for ppm of water (spec is <25ppm). It passes the spec, and if we flow out of a downstream tap it passes there as well. If we let it sit in the pipe for hours, it gradually builds up ppm of water at the tap above 25ppm. Flowing about 150–200 mls through the ¼” pipe gets back below spec.

Why this is confounding is the fact that the pipes and joints have been leak tested and the chemical is under about 10 psi. So how can the water vapor enter the system at the tap?