Sarcastic post:

I think we need to create a multi-tiered alert system that notifies users of the location of a police officer within 10 meter increments.

"NOTICE: An officer is within 50 meters!"

"NOTICE: An officer is within 40 meters!"

"NOTICE: An officer is within 30 meters!"

"NOTICE: An officer is within 20 meters!"

This way everyone can be sufficiently paranoid at all times, regardless of the officer's intentions.