This could be a number of things.
1. Simple bad wanding technique. Not keeping the tool horizontal to the carpet.
2. Underlay delamination. Goes powderey when failing.
3. Bad chems use, too high ph on wool.
4. Something the customer has used previously on carpet.
5. General dust blowing through gappy floorboards.
to name but a few, but you display the TACCA logo, surely this fundamental knowledge is vital to the criteria for membership?
I mean it does state on their website they are looking for quality carpet cleaner's only. Not having a go at you, but TACCA really needs to tighten their policy on vetting applicants!