You really don't know much about what police do. You should ask for a ride along with your local police force and find out.
I'll never forget the ride along my daughter and a couple of her friends were taken on by police as part of their 'punishment' for getting caught drinking. As the cop took them through the local trailer park he told the girls to roll down the windows. He said he wanted them to know how easy it was to sniff out pot use and keep an eye on the community. By all accounts my daughter never skipped a beat and asked why they couldn't do the same thing up on Belvedere Heights where the pot smelled better.