Guidelines in the marking of seized prohibited drugs

Dear PAO,

My brother was allegedly apprehended for selling prohibited drugs. A corresponding case was filed against him. He, however, had a different version. He revealed to me that the police had just brought him to the police, where he was made to point to a plastic sachet containing a white substance. His photo was taken in front of the plastic sachet and in the presence of a barangay official and the media. Then, the police proceeded to mark the plastic sachet. According to my uncle, who happens to be a policeman, the evidence should be marked at the place where my brother was caught selling illegal drugs, so the procedure adopted by the arresting officers is irregular. May I know the guidelines that should be followed or when and how the marking of seized prohibited drugs should be done? What is the consequence of non-compliance with the guidelines in the marking of evidence?