How to track someone using their iPhone 6 Plus without them knowing?

How to track someone using their iPhone 6 Plus without them knowing? - 1

No. Unless they know your login to your apple account. Then they can through find my iphone

Are you a stalker

If its your account you can use find my iphone. If not your account then no, iphones are way more secure then that.

No unless you had stealth tracker app like mspy installed on their iphone.