Yeha, he says it's from high up, so I imagine it's teh view distance, though you can never be sure, I am shocked to see people still using it...
In Hammer: Map / Map Properties / Max Viewable Distance : default 4096 - change it to whatever according to how far away you think the door is to where the player sees it, the number refers to grid sizes as well, so bear that in mind, if you add 16 you'll be able to see 16 units further hehe, probably wouldn't help you, but you never know!