The Management Engine (ME), part of Intel AMT, is a separate CPU that can run and control a computer even when powered off. The ME has been the bane of the security market since 2008 on all Intel based CPUs, with publicly released exploits against it, is now disabled by default on all Purism Librem laptops.
Source: Purism Librem Laptops Completely Disable Intel’s Management Engine – Purism
The idea is straightforward: Associate a series of ads with a specific individual as well as predetermined GPS coordinates. When those ads are served to a smartphone app, you know where that individual has been… It’s a surprisingly simple technique, and the researchers say you can pull it off for “$1,000 or less.” The relatively low cost means that digitally tracking a target in this manner isn’t just for corporations, governments, or criminal enterprises. Rather, the stalker next door can have a go at it as well… Refusing to click on the popups isn’t enough, as the person being surveilled doesn’t need to do so for this to work — simply being served the advertisements is all it takes.
Source: For Under $1,000, Mobile Ads Can Track Your Location – Slashdot