Note that the field is gone. Set down the insulative paddle on the ESD mat and measure the conductive plate again with the field meter. Note that a field is now present when you ve already discharged the field earlier on the ESD mat. This field was generated via field induction . As a material passes through or comes in near proximity to another field, a charge imbalance will occur from the presence of the foreign electric field.
Insulators are the biggest cause for charge induced from field induction.
There are two main ways to control this phenomena, air ionization and abstinence. Air ionization can be implemented using several types and models, depending on the application. Removing the insulative materials from the work place is the second method, but may not be practical for all applications, therefor air ionization is the universal solution.