Function departed_devices 

pub fn departed_devices(
    folder: &Path,
    key: &[u8; 32],
    memory: &Path,
    computer: &str,
    now_ms: i64,
) -> Vec<GoneDevice>

The devices that no longer count here (they left, you forgot them, another replaces them) and still have files in the folder: Settings lists each with “Remove its files”.