RF-blocking bags can also solve this problem.

Not... exactly.

Baseband processors could hypothetically remain energized and pull a list of nearby base stations, but it's not possible for it to be transmitting without you knowing (the battery drain would be extremely obvious). I have also never heard of this happening, nor can I find an example of this being documented behavior, nor can I find an example of someone enabling this functionality while the device has been powered off.

What can happen is something like the subversion of the power-off state to be simply an implementation of the OS's standby state - this is extremely difficult in any modern mobile OS (again I can't find examples of this being demonstrated without arbitrary code execution) but is the method rumored to be employed by the various security groups.

If you're being targeted by a group on the level of (ex:) the mossad, having a phone at all is a liability, even if the battery has been removed. If you're not the target of a security group, turning off your phone is a great way of preserving your privacy.

(Also, fun little side note, depending on your region leaving your phone at home won't do anything to prevent your location be tracked. Whee!)

You can put it in a faraday cage