[Vortigern78]

As title really!

I'm wondering if I get a second hand phone could the previous user track me after a full reset?
But also conversely, if my phone got stolen, they would have to reset it to get past security.... but would that leave me unable to track it?!

[Harrie-S]

The "Find my Phone" works only in combination with the MS account on the phone and if you allow it in the phone settings.

So if you get a second hand phone and set it up with your MS account the previous owner can not track you anymore.
If the phone gets stolen and the thieve changes the MS account (or hard reset it) you can not track it anymore but at least they can not get your data but they can "reuse" your phone. But you can inform the police or your carrier to "block" the IMEI so it can not be used as phone anymore.

Now in W10 there will also be a reset protection which prevents a hard reset but it is very unclear if this works in current phone. (In the w10 Technical Preview I can enable the reset protection but I have no idea if this really works).

[noersetiawan]

Find My Phone require an MS account is logged in on that phone and it has access to internet. There's no legal way to track phone using IMEI (unless you're FBI and the likes). On older Lumia phone that doesn't have reset protection, a full reset will remove everything, including the logged in MS account, thus make the phone can no longer be tracked. On newer phone, even if the thief reset the phone, they will not be able to bypass security unless they have your MSA credentials.