Did my computer unlock itself?

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.