[gvb]

If your phone is powered off or in airplane mode it is not supposed to emit RF and thus cannot be tracked. This is a matter of trust, so if your threat model includes high end threats, the assumption that it follows the normal requirements may be invalid.

If you remove the battery, it will be unpowered and unable to emit RF and thus cannot be tracked. While it is theoretically possible to hide an auxiliary battery in your phone, that would be very hard to achieve, especially in modern thin phones. If your threat model includes highly motivated state sponsored actors, this is could be achieved.

If you put your phone in a RF-tight enclosure (e.g. metal box), the RF energy cannot get out and thus it cannot be tracked.

[aembleton]

Modern thin phones don't let you remove the battery, so they could easily continue to transmit RF.