The change in lighting is the only practical way to find it but you also have to get lucky. The easter egg is something (relatively big) that spawns inside a certain radius from your spawn point in one of the rooms. It's most likely not going to spawn close enough for you to see it immediately (but it might) so you'll have to get lucky and start walking into the right direction (without knowing it at first) until you see a change of lighting. If this wasn't a just a simulation you'd also feel a change of temperature...