You suspect a Man-in-the-Middle (MitM) attack inside the network.

CEH v13 identifies ARP spoofing/poisoning as a primary MitM technique in local networks. In such attacks, a single IP address maps to multiple MAC addresses, indicating ARP table manipulation.

This anomaly allows attackers to intercept traffic between victims and gateways. Increased traffic or DNS activity may occur but are not definitive indicators. Thus, IP-to-MAC inconsistencies are the most reliable confirmation of MitM activity.