weird windows event log error help

Our network security guy called me about an event error found on our windows domain controller which also doubles as our iseries file server (qntc).

Here is the error.

AV - Alert - "1608147140" --> RID: "18116"; RL: "9"; RG: "windows,authentication_failures,"; RC: "User account locked out (multiple login errors)."; USER: "(no user)"; SRCIP: "None"; HOSTNAME: "(Host-x-x-x-x) x.x.x.x->WinEvtLog"; LOCATION: "(Host-x-x-x-x) x.x.x.x->WinEvtLog"; EVENT: "[INIT]2020 Dec 16 14:32:17 WinEvtLog: Security: AUDIT_SUCCESS(4740): Microsoft-Windows-Security-Auditing: (no user): no domain: xxx.xxx.com: 0x8000000000000000 message: A user account was locked out. Subject: Security ID: S-1-5-18 Account Name: NT5$ Account Domain: X_DOMAIN Logon ID: 0x3e7 Account That Was Locked Out: Security ID: S-1-5-21-1175146227-807941459-751859383-501 Account Name: Guest Additional Information: Caller Computer Name: XXX [END]";

The part that is confusing to us is the "no user". How can the iseries access a windows server with no user?

Any thought as to what might trigger this event?