Have you ever wondered why Roku TVs are so cheap? Check the default privacy settings. They fingerprint every single thing that displays on the TV and phone home. Here's an engineer right here on HN talking about it. https://news.ycombinator.com/item?id=21012268

Also, there are ultrasonic beacons and other such signals at any reasonably busy public place and in many places you wouldn't expect. https://en.wikipedia.org/wiki/Cross-device_tracking