APERTURE'S HOUR fills the gap between Portal & Half-Life by giving a look at Aperture Laboratories during the Black Mesa Incident and the resulting Seven Hour War. Taking the role of an employee stuck in a testing cycle following GLaDOS' complete takeover on Bring Your Daughter to Work Day, make your way through and escape the now malfunctioning facility.
Here's three images for you to look at.
Enjoy.
And that explains how Doug could travel through the complex without being spotted.
haha, interesting idea.