Strange logon activity for Administrator in Event Logs

On one of our public facing servers the Administrator account logged in at 6:45am GMT. It wasn't a member of staff.

Details from the event logs

1st event
        Logon attempt by:   MICROSOFT_AUTHENTICATION_PACKAGE_V1_0
        Logon account:  Administrator

2nd event
        Logon attempt using explicit credentials:
        Logged on user:
            User Name:  S15252541$
            Domain:     WGS15252973
            Logon ID:       (0x0,0x3E7)
            Logon GUID: -
        User whose credentials were used:
            Target User Name:   Administrator
            Target Domain:  S15252541
            Target Logon GUID: -

        Target Server Name: localhost

3rd event
Successful Logon:
    User Name:  Administrator
    Domain:     S15252541
    Logon ID:       (0x0,0x73837CF)
    Logon Type: 4
    Logon Process:  Advapi  
    Authentication Package: Negotiate
    Workstation Name:   S15252541
    Logon GUID: -
    Caller User Name:   S15252541$
    Caller Domain:  WGS15252541

4th event
Special privileges assigned to new logon:
    User Name:  Administrator
    Domain:     S15252541
    Logon ID:       (0x0,0x73837CF)
    Privileges: SeSecurityPrivilege
            SeBackupPrivilege
            SeRestorePrivilege
            SeTakeOwnershipPrivilege
            SeDebugPrivilege
            SeSystemEnvironmentPrivilege
            SeLoadDriverPrivilege
            SeImpersonatePrivilege

5th event
User Logoff:
    User Name:  Administrator
    Domain:     S15252541
    Logon ID:       (0x0,0x73837CF)
    Logon Type: 4

I've changed the Administrator password as a precaution, should I do anything else or am I worrying unceasingly?

p.s. This isn't an April fools