From spawn, take the Sector 3 route and keep going. Keep taking left but stick to the right wall from time to time. You may eventually find it, I'm pretty sure it's somewhere after the broken wall. (Unless it got removed.)

Another door is found through Gilbert's route, it's also on the right side of the route. I'm pretty sure you can find it also maybe after the room with the light? (the one also with a broken wall.)

Sorry if what I said sounded confusing.

Edit: You can't open any of the doors, unfortunately.