windows 8 - Did my computer unlock itself?

I woke up around noon to find my computer unlocked, despite having remembered locking it the previous night and not using it since. I checked the security log, and saw a "logon" event from 11:16 AM. Only thing is, I was asleep then, and noone else with access to my computer knows my password. Just to be safe, I'm running a virus scan, but so far it hasn't found anything. What could have happened?

The text of the event log entry is below.

By the way, I checked and there is a "logoff" event around the time I remembered locking my computer last night.

Log entry:

Log Name:      Security
Source:        Microsoft-Windows-Security-Auditing
Date:          1/29/2014 11:16:10 AM
Event ID:      4624
Task Category: Logon
Level:         Information
Keywords:      Audit Success
User:          N/A
Computer:      FLARNDT
Description:
An account was successfully logged on.

Subject:
        Security ID:            SYSTEM
        Account Name:           FLARNDT$
        Account Domain:         WORKGROUP
        Logon ID:               0x3E7

Logon Type:                     5

Impersonation Level:            Impersonation

New Logon:
        Security ID:            SYSTEM
        Account Name:           SYSTEM
        Account Domain:         NT AUTHORITY
        Logon ID:               0x3E7
        Logon GUID:             {00000000-0000-0000-0000-000000000000}

Process Information:
        Process ID:             0x188
        Process Name:           C:\Windows\System32\services.exe

Network Information:
        Workstation Name:      
        Source Network Address: -
        Source Port:            -

Detailed Authentication Information:
        Logon Process:          Advapi  
        Authentication Package: Negotiate
        Transited Services:     -
        Package Name (NTLM only):       -
        Key Length:             0

This event is generated when a logon session is created. It is generated on the computer that was accessed.

The subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe.

The logon type field indicates the kind of logon that occurred. The most common types are 2 (interactive) and 3 (network).

The New Logon fields indicate the account for whom the new logon was created, i.e. the account that was logged on.

The network fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases.

The impersonation level field indicates the extent to which a process in the logon session can impersonate.

The authentication information fields provide detailed information about this specific logon request.
        - Logon GUID is a unique identifier that can be used to correlate this event with a KDC event.
        - Transited services indicate which intermediate services have participated in this logon request.
        - Package name indicates which sub-protocol was used among the NTLM protocols.
        - Key length indicates the length of the generated session key. This will be 0 if no session key was requested.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="Microsoft-Windows-Security-Auditing" Guid="{54849625-5478-4994-A5BA-3E3B0328C30D}" />
    <EventID>4624</EventID>
    <Version>1</Version>
    <Level>0</Level>
    <Task>12544</Task>
    <Opcode>0</Opcode>
    <Keywords>0x8020000000000000</Keywords>
    <TimeCreated SystemTime="2014-01-29T16:16:10.375881200Z" />
    <EventRecordID>96945</EventRecordID>
    <Correlation />
    <Execution ProcessID="380" ThreadID="8756" />
    <Channel>Security</Channel>
    <Computer>FLARNDT</Computer>
    <Security />
  </System>
  <EventData>
    <Data Name="SubjectUserSid">S-1-5-18</Data>
    <Data Name="SubjectUserName">FLARNDT$</Data>
    <Data Name="SubjectDomainName">WORKGROUP</Data>
    <Data Name="SubjectLogonId">0x3e7</Data>
    <Data Name="TargetUserSid">S-1-5-18</Data>
    <Data Name="TargetUserName">SYSTEM</Data>
    <Data Name="TargetDomainName">NT AUTHORITY</Data>
    <Data Name="TargetLogonId">0x3e7</Data>
    <Data Name="LogonType">5</Data>
    <Data Name="LogonProcessName">Advapi  </Data>
    <Data Name="AuthenticationPackageName">Negotiate</Data>
    <Data Name="WorkstationName">
    </Data>
    <Data Name="LogonGuid">{00000000-0000-0000-0000-000000000000}</Data>
    <Data Name="TransmittedServices">-</Data>
    <Data Name="LmPackageName">-</Data>
    <Data Name="KeyLength">0</Data>
    <Data Name="ProcessId">0x188</Data>
    <Data Name="ProcessName">C:\Windows\System32\services.exe</Data>
    <Data Name="IpAddress">-</Data>
    <Data Name="IpPort">-</Data>
    <Data Name="ImpersonationLevel">%%1833</Data>
  </EventData>
</Event>

I don't know of any programmatic way to unlock a workstation from say a running program (like a virus, unless it seriously compromised the Windows security stack). Other than someone at the keyboard, that leaves the possibility of remote-control software that can send keystrokes, which is sometimes included with viruses. Even with this scenario the attacker would still need to know your password.

Look specifically for events sourced by "Security-Auditing" with Event ID 4800 (lock) and 4801 (unlock). Those will directly correlate with the actual locking and unlocking of your session... The other logon/logoff events are frequently generated in the background for things you don't necessarily expect.