[From pp.58-59 of Cop in the Hood]
To meet the standards needed for a formal prosecution, one must follow the informal rules imposed by the state’s attorney. Rule number one is don’t take your eyes off the drugs. Drug charges against a suspect will not be prosecuted in Baltimore City if an officer fails to maintain constant sight of the drugs. A suspect fleeing from police will throw down drugs while running. An officer in foot pursuit must then choose between catching a suspect with no drugs and retrieving the drugs with no suspect. Officers generally will choose to follow the suspect over the drugs because— along with a personal desire to catch a fleeing suspect—arrests are a police statistic used to judge performance. Found drugs are not.
After catching the suspect, the officer will return to retrieve the drugs and charge the suspect with possession, knowing full well that the charges will be dropped if the report is written honestly. But officers are rewarded for arrests, not convictions. If the drugs can’t be found--lost in weeds, scooped up by a bystander, or never there to begin with--the officer is in a bit of a bind, left with the noncrime of “felony running.” You can’t lock somebody up for drug possession without drugs. And after a chase, even loitering doesn’t apply. But the officer will find some crime, however minor If you run and get caught, you’re probably not sleeping in your own bed that night.
[This is why Freddie Gray was arrested for a barely illegal knife].