Strange watermark on a iPhone photo taken in Target
I would be willing to bet that Target is using hidden watermarks that are only visible when taking a photo (similar to the watermark that is used by banks to protect checks).  What I hypothesize they are using is a fluorescent dyes that only shows up under certain lighting conditions (like the high intensity led of a smartphone camera flash)

See:  Invisible glass watermarks will ensure your new watch or smartphone is genuine

Out of sheer curiosity, I would do a simple test: take a compact mirror into the restroom with you.  With your phone in Airplane Mode, take a picture of the bathroom mirror and of the mirror you brought in.  If the image is in both places, it's "tech" related.  If not, it's in the mirror.