The Industrial Control System (ICS) encompasses various control systems, industrial equipment, and instrumentation critical to manufacturing processes. As cyber threats have exponentially increased in recent years, large organizations allocate substantial portions of their budgets to safeguard sensitive information. One effective security measure is the use of air-gapped systems (isolated networks), which are intentionally isolated from unsecured networks. However, despite their perceived reliability and uniqueness, air-gapped systems (isolated networks) remain vulnerable to attack vectors that can lead to significant financial loss and physical damage. This paper explores various methods attackers use to establish covert channels to infiltrate or exfiltrate sensitive data from isolated ICS environments. These methods are categorized into acoustic, electromagnetic, optical, magnetic, power, and thermal techniques. Additionally, the paper presents realistic attack scenarios and offers countermeasures to prevent and detect data leakage in industrial air-gapped systems (isolated networks).