content: Anything not connected to the internet, like a encrypted hard drive.  Eh, there is that neat virus that seeks out air-gapped equipment... then uses the speakers to and the microphone to communicate in the ultrasonic range if and when it finds a "control" machine... Could be in your usb thumb drives microcontroller right now, if you are interesting enough to a large government. Or even if you are a nobody at an interesting large company.