Odd situation

Had a prior client call. Moved into their home last July (07) Noticed the young child (3 yrs.) would get dirty feet when running around the carpeted house all day. So they had it professionally cleaned. No help, still would get a Grey like coating on her feet, so they had it cleaned again (foam type)) toward the fall season. This seemed to work through the winter season (forced air heat) Didn’t notice any problem unit this spring again.
They had two carpet people come out, one who lifted the carpet but didn’t see any issues.
Looking for ideas on what might be causing this. thanks!

Crawlspace? or basement? was it finished?
and where is the return air for the furnace?
They check the filter for the furnace?
A CO and Noxious gas detector would give some peace-of-mind.
A few more details might help…

Take a sample of the Dirt (gray stuff) and have it analyzed. If it returns.