This will be an explanation on how to reach an easter egg hidden in "The Lab". Perhaps you already found it while messing around (like I did) but it's easy to overlook, so here's a guide.

When you play the lab for the first time, it shows you the mountain landscape first. Once you actually enter the lab, you can go back to this environment using the postcards on the wall exhibit.

You may have noticed there's a cable hanging on this wall that is connected to a socket. There are multiple sockets here, one for each picture.

The rest of the exhibit looks like a broken section of wall from a house. There's even some pipes and plumbing visible...and a valve. A big red valve. Which happens to be logo of the company that developed The Lab. If you look closely, the valve's axle is in fact another socket.

Grab the cable, unplug it from the picture it is connected to, and plug it into the valve by using the trigger button like you would for a portal sphere.

Once you have plugged it in securely, you can put the nearby portal sphere on your face and you will be inside the demo room Valve used for early Vive prototypes. In fact this seems to be the room they used when demoing the Crystal Cove prototype.

You can hear ticking, it's coming from the object shaped like a fire alarm near the door of the room. It is in fact a VR alarm and if you pull it down, you can use the ping pong paddle from the desk to deflect the balls that will be fired at you.

After entering the first demo room you should have a good look around.
Don't portal back to the lab just yet when you get bored though, because there's a prototype headset lying on the floor...and you can pick it up!

Much like a portal sphere, just put it on your head like you would with a headset and try to see through the lenses. Turns out it takes you the second demo room!

There's some stuff to do in this room, for starters you will meet one of the robots from Portal 2 co-op face to face. The one you can also try to repair :) . There's also a clipboard you can carry around. The most interesting thing is probably that you can hear ticking again. Another VR alarm, this time on the pillar near the desktop computer. This will give you something to do. Try to annoy the robot, he actually reacts to your actions. If you pull the trigger you get a sharp pointy thing, squeeze the grip buttons underneath your controllers to shoot the lasers (which you might recognize from the Vive tutorial).