If the napkin were just a napkin, it would have been tossed in the trash and forgotten about two seconds later.
Or the Realtor was doing what they do, keeping in contact with a customer and showing attention to detail (and likely was more emailing about the pillows than a black and white embroidered handkerchief.)
Either way, that is a bit of a side question. For the Pizza Gate theory to stand up, Sheldon has to know what is on a handkerchief which is hours away, that she’s talking to a pedo, that he’d understand the code, and how it got stained. And be be comfortable enough adding an aide into the conversation.