Because the rune isn't in the room when you first arrive, the green haze that shoots lightning is really just a marker to say "remember this room". So that the player would go back to something they remembered after completing the cross keys part, rather than just searching everywhere for the last rune.
As Terkio says... There will be no absolutely definitive map, because in certain parts there are 50/50 scenarios. In some of these (though not many), the layout itself is different and not just objects/monsters. The majority of the map is always the same, but because of these areas each play will be slightly different.
Because they're all 50/50, like flipping a coin, players will get many of the same scenarios... but because there are quite a few of these areas spread out over roughly the first 75% of the map, the chances of getting the same dungeon twice is extremely low.
The scenarios are decided by rapidly cycling silent triggers spread around the beginning part, so you cannot know which ones you've hit.
He's also dealing with windows 10
.
