Depending on how careful you are and what equipment you've got on hand, you might be able to detect that going on, but I think "easily" is a stretch.

If there is a secret involved in this it would be undocumented commands sent by the network to phones that then interpret them as commands to connect to the network, plus baseband processors the periodically look for such commands. They don't radiate, so very little power is used, but they should be entirely dormant.

I ask this all the time but have yet to find anyone that has actually measured it. According to Snowden, basebands are still powered and operational even when the main application OS (android/ios/etc.) are shut down. And a non-removable battery sure doesn't help that. Plus, he also says that the baseband often has its own separate connections to the GPS, microphone and camera and can operate them without the application OS involved.

If the baseband is still powered, I can only assume it is simply sending (or responding to) periodic commands that do not require network authorization, which could possibly still let the network know where you are even without involving any roaming charges.