Windows 7: Security Breach?

Hey,

this has happened twice to my Windows 7 x64 Ultimate SP1 this week;

I usually start my computer in the morning. My profile is an admin account, with quite long and hard password.

I often forget to log in to the account, yet I just leave the computer running (stuck in the loading screen). Twice this week, when I have returned from the "standby mode" by pressing enter (or random keys), my Windows freezes for 3-5 seconds, and after the freeze it forces a log-in, putting me inside the account. So no password is required.

Is this a known issue? My computer is a table unit so no strangers should get into it like this, but if this happens on laptops shouldn't this be called a security breach?

I have ran several anti-virus scans (F-Secure, Microsoft Essentials), as well as Spyware Terminator - but found no malware / viruses.

Thanks in advance,
Otto