Function heard_of 

pub fn heard_of(
    folder: &Path,
    key: &[u8; 32],
    memory: &Path,
    computer: &str,
    device: &str,
) -> i64

When device was last heard of here, from sealed facts only (last_heard: its latest entry read here, its records’ last clock read), in milliseconds; 0 when nothing is known. What a replacement says of it (devices::Replaced::heard).