"To alert the police" means to inform or notify the police about a problem, crime, or emergency situation so that they can take necessary action. Full definition
Regardless of how you paid for your gifts, you should always file a police report to help alert police of the incident and to potentially prevent this crime from happening to someone else. (comparisonmarket.com)
Victims of online stalking are supposed to immediately alert police and providers of Internet services and sites where the threat arose. (womensenews.org)
The bill would require 911 to be called to alert the police of a possible crime. (dailyfreeman.com)