It's a common bit of firearm carrying wisdom. Not directly equitable to, but in the same vein as, "even when the gun is unloaded, its loaded". Of course its a

It's a common bit of firearm carrying wisdom. Not directly equitable to, but in the same vein as, "even when the gun is unloaded, its loaded". Of course its a deterrent, but the only thing that makes it more of a deterrent than a whistle is that the officer is (or presumed to be) ready to shoot.