On our cars I'm sure that the tailgates sound an alert, and the motor/struts make a noise too, so you'd know it was moving even without an alert, unless the person is hard of hearing or in a very noisy environment.
In my experience your feet have to be deliberately moving and well beneath the bumper, and so it's not a natural position for standing in when loading/unoading, and as such requires a deliberate action to trigger.
In fact I find that getting the thing to blooming activate is a bigger issue, but maybe I just have small feet and long arms.