Similar to @ccmac33 , I would support that it is more likely a partial cos play costume.Many have wondered about the long white robe the perp is wearing. Some have speculated it is a bathrobe or even Princess Leia like costume. I am wondering if it could actually be some sort of plastic tarp like shielding to keep blood/dna off perps clothing.
It would not need to be Princess Leia / Star Wars per se. Rather, some cos players might participate in two or more different theme worlds. Fore example, Star Wars and Rennaisance Faire type activities.
As a side note, years ago I was interested in a young woman who participated in the Society for Creative Anachronism ( mixes historical medieval with Lord of the RIngs type fantasy type costumes). I Got..... turned down cold.
Anyways, and more to the point, she and her friends were crafty and sewed their own costumes, cast silver jewelry and built small props.
It was not unheard of for members of the group to wear partial costume around campus. None of them were blurring reality and fantasy. Rather, they liked say, "Elfin fashion" and liked showing their creations to theatre majors and other counter cultural types.
I would not be surprised if some wore partial costume on occasion with out even thinking about it and with no active intent stop by the theatre hang out spot. It could be the same with the perpertrator.