Our personal data is being collected every day by the technology we use. We know there are privacy terms (that many of us doesn’t read, just accept them) telling us the use that they will give to our data, and maybe they use it for another purposes that are not established in the terms. In other words, the companies that we granted the access to our data know who we are and what are we are doing, and maybe they’re sharing it with another companies. Remember that fantastic clothes you were searching one hour ago, that was transformed in a recommendation add about that items linking to an online store (probably Amazon), or searching about cool places in a city in Europe that became in an add linking to a page where you can get tickets to travel. Look into the section «My Activity» in Google Maps to know where you were being at a certain time.
That data was capture probably using a device (smartphone or computer), now imagine more data being capture as you move, what places you like to frequent more, what kind of transportation you use more and what streets you use to move and being categorized as a «probably burglar» or a «good citizen», well now that’s possible and is being tested in the streets of Amsterdam, it is being used to reduce the number of crimes in the city and to make them more secure.
The data is being capture using small traffic lights that detect the MAC address of smartphone or the device you have connected to the internet. You can say you are smart enough and leave the device at home, but probably the identify you using facial recognition with the smart cameras of the city, and let’s be honest, we can live without our smartphone, everything is so connected nowadays that we have to use our gadgets to do (or even in an easier way) a lot of things.
You can see this article for read more about this.