Secret watermark revealed when copied?

We went to an Old Time photo place in Branson (Busters Old Time Photos) and they told us if we scanned or took photos of the prints we bought that a watermark would appear and ruin the print. They said even an iPhone photo with no flash would ruin it. Is it total bullshit or truth? I just want to show my mom the photo (we live in different states) and don't need to copy since I bought the print, but don't wanna ruin my print if it's true. She said something about the paper having a layer of glue and reacting with lumens when your iPhone focuses. Sounded like a crock of **** if you ask me and a ploy to get us to buy more photos. Thanks for the help.

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