The machines do have varying, selectable levels of intensity of screening. Any normally used setting will on most occasions trigger an alarm. They can be set to exempt or be less sensitive to different levels of the body so it is not unusual for the shoe setting to be lower than elsewhere on the body.
Which i find so amusing as when you ask them they always say that they are set on the same settings and all calibrated properly. However, and im sure that many of you have done the same, on the same day wearing the same clothes etc, when i go through HBA there isnt even the slightest noise, then you go through MEL and i basically have to nude up to get through, but SYD all i have to do is take off my belt! You cant tell me that they're not all the same in that instance.
As an aside i often find MEL to be the most sensitive but i don't know if thats me.