Usually (to prevent such a situation), there is an SOP specifying the use of a %Recovery or Correction factor below a certain %Recovery. Also, there should be in the ‘Site Cleaning Validation Master Plan’ a minimum 1.0% surface area for critical process equipment.
However, the question remains. There are ‘materials of construction’ that absorb the API because there are not inert. An example would be the cloth (like polyester) that is used to manufacture chairs. Thus you will NEVER be able to demonstrate effective cleaning below the acceptance criteria. Is the your PETG woven or a sheet?