The carpet was clean when the machine left and gray again by the next morning. There are five causes, they look almost identical, and one of them is not dirt at all.
Carpet holds far more soil below the surface than on it. When a wet method puts moisture into the pile, that moisture travels down to the backing, dissolves what is sitting there, and then carries it back up as the carpet dries, depositing it at the tips of the fibre, which is exactly where you look.
The tell is timing. If the marks were gone when the crew left and came back as the carpet dried, that is wicking and not a failed clean.
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