Every movement with a gun sounds like there's a loose screw in it (it always clicks). Also it usually has a clip of 300+ bullets.

Every mouse or keyboard input into a computer, every loading bar, every screen popping up makes screaching sounds. Except when having a failing DVD drive or broken hard disk I've never heard any computer making these sounds.

A secret tracking or listening device has a blinking red light and beeps.

Every car, always with airconditioning, drives with open windows because of the window reflections. Even during rain, extreme heat or highly contagious zombies trying to bite you through the open window.