1. At the start of the game, when you are banging on the door, turn around. After a bit Gordon Freeman from the original game should go past on a monorail train.

2. In the security room in the guard's complex, look at the screens. You'll see him again, walking past those spinning signs.

3. At the end of the game, as you suddenly get teleported to all of those different places, you will arrive in a storeroom. Look through the air vent up on the wall. You'll see him again, being dragged by two soldiers, and you'll hear them say stuff from the original game after Gordon gets captured.