Almost every one of us have a smartphone today. This is the most basic way to track someone as it just require an active internet connection and GPS feature turned ON. Everyone is smart enough to disable their internet connection and GPS service only a handful of technology-abled people might get caught.
ISP stands for Internet Service Provider, they are the company whose SIM cards you use to make calls, SMS. Police can get warrant against you and use it to retrieve information from your ISP about the last location you were connected to any of their towers. Whenever you move from one location to another, you keep on falling in different tower parameters and ISP always keep record of their clients connection. So, in this way police can track your location.
TRIANGULATION: It is the method using which authorities can track your most accurate location. In most of the place around the globe we're always connected to multiple towers simultaneously (16-17 generally). Police can triangulate you between the these towers based on your connection strength and proximity.
IMEI number is an unique number given to every phone. Multi SIM phones have multiple IMEI numbers. If someone tries to remove SIM card and continue using the phone then each time you get connected to network, police will be pinged with your location.
The documents you submitted while purchasing SIM card can be accessed by police to know your address.
Android phones have a built-in feature to store their location history in cloud storage. Police can access your location history from google and predict your future locations.
These are the major ways police tries to track down the suspects/criminals. There are even more powerful and advanced ways of searching, but they're only used in extreme cases. Facial recognition at public places is one such method. The best advice is to stay away from police involvements in any condition.
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