A network administrator for a completely air-gapped and closed system has noticed that anomalous external...

Reviewing video from IP cameras within the facility would be the most likely process to expose an attacker who has compromised an air-gapped system. Since air-gapped systems are isolated from external networks, an attacker would need physical access to the system or use some covert channel to communicate with it. Video surveillance could reveal any unauthorized or suspicious activity within the facility that could be related to the attack. Verified References:

https://www.welivesecurity.com/wp-content/uploads/2021/12/eset_jumping_the_air_gap_wp.pdf

https://en.wikipedia.org/wiki/Air-Gap_Malware

https://www.techtarget.com/searchsecurity/essentialguide/How-air-gap-attacks-challenge-the-notion-of-secure-networks