That's is tricky for older PT employees there is 2nd shift pay, this benefit is not available to newer employees(not sure of the cut off)
now if you have gone home and are "called in" you should get call in pay, which is basically a minimum.
I've been asked/called in, to check the alarm at the building on occasion
and I'm paid call in pay which is 2hrs(I believe) but since I live really close its worth it to me for 20minutes of my time..
I'm sure others will have different experiences here,*I look forward to reading what others have to say.