CommonWealth Beacon takes a look at the controversy over ShotSpotter, the expensive system police in Boston and other cities use to locate the source of potential gunfire: BPD Commissioner Michael Cox stood by the system at a Monday hearing, but critics say it has a large number of false positives, which means people in the minority neighborhoods where the sensors are located are more likely to be grilled by police investigating false leads. Chicago announced earlier this year it's abandoning the system.