Occasionally there are reports about eavesdropping via phones which are switched off or in offline (or airplane) mode. People put their phones in the fridge or remove the batteries to prevent such eavesdropping.
Have there been proven situations where something like that happened? What phones were used for this? Did the phones have modified hardware, or was only the software altered?
How could one detect such eavesdropping?
I'm aware that some software on the phone could collect data (e.g. record a conversation) and upload the content only when the connection to network is established or the phone is switched back on. But my question is specifically about the situation where the phone would be online although the user would think it's switched off or in offline mode.