We are coming up to that time of year where cleaners sometimes suffer from callbacks due to carpets remaining damp and starting to smell.
This is due to the high humidity in the air which prevents any more moisture from the carpet evaporating off.
This allows the moisture to remain in the base of the carpet for a long period, often days and results in the growth of invisible moulds and mildews which is what causes the musty odour.
This is especially prevalent on empty rentals where you clean and then shut the door behind you.
Two ways to combat this, one is to rinse with a suitable product which will prevent any moulds or mildews to grow. Suitable products are Clensan, Microkill or Formual 429. These are broad spectrum biocides which deal with Bacteria, Fungi and Viruses.
Ordinary deodourisers usually only deal with Bacteria, so are not suitable to combat this problem.
The second way to deal with it is to force dry using airmovers, Dripods etc.
If you do suffer from an incident of this nature, don't be tempted to re-clean the carpet or just spray a deodouriser, this will only increase the moisture and make things worse.
You can advise the client to air the room thoroughly and it will eventually dry out, once dry, the mildews etc will die off and the smell will go. You could also spray one of the products mentioned and force dry or just force dry.