Did you lift the carpet to determine if the backing was cellulose??
If it is, it is a good chance it is cellulosic browning, removed with Browning Remover, NOT Browning Treatment.
Browning Treatment will help prevent cellulosic browning.
Browning Remover, has a higher % of H2O2 (or maybe over there, Acetic Acid), and is effective at removing the problem.
The plastic mats exacerbate the problem.
Plastic will retain moisture UNDER the mat, keeping the carpet damp & allowing mould spores to form.
Even in cases where the carpet may be dry, in times of very high humidity due to extended rainfall, it is not uncommon for moisture to gather under the plastic & start the problem.
Many times the air con; will fix this, BUT on weekends when the a/c is not on, is when the problem can start.
This is what I've found living in the tropics, don't know where you are, so cannot qualify my findings for you.
