The Halloween racks at thrift stores seem to be filled not just with used costumes (as you can see in the photos on the left), but also with Halloween t-shirts, articles of clothing that seem passable for a costume (but you're just not sure what they could be used for) and graduation robes. I'm sure one or more of those items are hidden in these pictures.
I once wondered why graduation robes get thrown on these racks. Then I guessed that these could actually be used for something. White ones could be used for angels, and red ones could be used for devils. One year, I decided to be the half-angle/half-devil (also called other names such as "Heavenly Devil"). I bought two thrift store graduation robes (one white and one red, of course) and cut them each in half, sewing each half down the back. I made a devil cape from the other half of the red robe.
EDIT: I got ahead of myself. I went down to the Goodwill about an hour after originally posting this. The wizard robe is still there.