A few weeks ago, I went into a Lenscrafters because I needed some cleaning wipes for my glasses. I have Wifi, Bluetooth, and Location services disabled for Facebook.

Someone messaged me on Facebook Messenger, I responded, put my phone back in my pocket, and finished the transaction, and then left Lenscrafters and walked down to the store that I was actually in the mall to see.

Within the day, I started seeing contextual ads for eyeglasses in my facebook feed, along with ads in other apps for eyeglasses.

I'm only suspecting that there's a service that does some flavor of ultrasonic beacon/listening technology, and facebook (along with DFP and the other large ad markets) are buying that service in troves.