Photo-Tom:
With this:stepper.moveTo(100);
stepper.moveTo(-100);
my thinking was that when the pir is triggered HIGH the stepper will move 100 steps clockwise and then after that 100 steps counter clockwise.
This idea is all wrong - sorry. You still seem not to have grasped how the AccelStepper library works.
Those two lines behave as if you said move 100 and then changed your mind and said move -100. When the code gets to the line stepper.run() (which is where the actual movement happens) the only value that will be seen is the -100.
...R