You certain an unpowered phone can not be tracked? I am not. The only way to be sure you are not being tracked is to have your phone in a rf proof box.
If there is no power, there is not tracking. It is not easy to ensure that there is not power.
Even if you take out the battery (very difficult with iphones) there are potential other power sources such as capacitors.
A Faraday cage is the best to insure that no tracking is ongoing. Another advantage of a Faraday cage is that it prevents reception of GPS. Some phones may be sophosticated enought to recieve GPS signals, store the GPS locations, and send them to catch up on the tracking.
I do not think that is common, but it is possible.
Or to remove the battery. But the newer Android phones no longer have that capability.