Is it illegal to track someones phone without them knowing?

Related questions

1. How can I track someone's location?
2. Can I track someone's phone without them knowing?
3. Can private investigators use GPS tracking?
4. Is it illegal to put a tracker on someone's car?
5. How can I track my sons iPhone without him knowing for free?
6. Do tracker blockers work?
7. Do magnets affect GPS tracking devices?
8. How long does a GPS tracker battery last?
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Tags

1. #consent
2. #through
3. #technically
4. #legal
5. #enforcement
6. #person
7. #usually
8. #without
9. #agency
10. #track
11. #phone
12. #adult
13. #someone
14. #illegal
15. #physical
16. #though
17. #unless
18. #warrant
19. #location
20. #possible
21. #always