Strange watermark on a iPhone photo taken in Target My friend recently took a photo with iPhone 5 in Target's changing room and a watermark automatically applied over it saying "You look goooood. #UniversalThread #TargetStyle". 
It wasn't written on the mirror, it's clearly watermarked on the photo after the capture.

How did Target even do that? It raises some serious privacy issues. Data/wifi was off during the taking of the picture and the stock camera app was used. I found zero info about it online.
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.