I’m not sure of the specific case numbers but searching brings up many sources, e.g. https://www.cnet.com/news/privacy/fbi-taps-cell-phone-mic-as-eavesdropping-tool/ (2006)

Even if your phone is OFF, it can still track you from the towers and transmit your audio (this is documented in court cases bringing down the mob). It’s built into the chips themselves, at a lower level than the operating system. Do not bring your phones to protests.