Tweets are tracking devices, and so are Facebook posts, and Google Plus comments, and text messages and on and on.
The same technology that consumers have embraced to allow them to find products and get recommendations more efficiently often has geotagging. That picture you took so your cell phone probably has the location embedded in the header.
The phone itself, even when in standby mode, pings towers to let them know that it is available to receive calls. That iPad is retaining information about available wifi networks, even if it appears to be unconnected.
The same technology that can be used to find someone, can also be used to recreate a historical set of movements. That history is your device confessing for you.