I actually never noticed that.
I quite agree with Sarah. There are so many spells we do not know about, and many can cause fog or darkness. Plus, the students would be angled- they aren't seeing anything from a direct central point. They surround the maze and look down at angles. And because of that, it can be hard to deduce what exactly is happening.
As for Dumbledore trusting Moody- it really makes me wonder if he trusted him completely. Dumbledore knows a lot of what happens in the school, and he seemed to have allowed Harry to take on Voldemort in the first book. He even told Snape to keep an eye on Quirrel. Can we be absolutely positive that Dumbledore didn't have an inkling? Still, I'm sure he knew that something similar to the graveyard scene would happen sooner or later. :)
Anyways, back to the real question. I just remembered omnioculars!!!!!!! They had them for the Quidditch match- would students have them for this task? If they did, it might not be so hard to see what was happening...
Well thats a really goo question so here are my two knuts. :)
Since we all seem to agree that a large amount of time passed I believe the teachers got worried and did a spell to see if cedric and harry were still in the maze or harmed in a very severe way so they couldn't shoot sparks. After reallizing there was no one left in there (Krum and Fleur had been removed) they magicly vanished the hedge. .ONce vanishing it the stadium grew loud with wondering voices and, as I recall, immedially went silent when harry showed up with a dead Cedric.
concluding this my theory is the maze and dangerous creatures inside it were vanished by Dumbledore.
Have a Great day!