Rockslider has put up another piece on Halo 1; this one covers how to lure Banshees into areas they don't normally belong, and the strange behavior they exhibit once they get there. This piece covers level 2, "Halo".
Quite what the explanation for the weird behaviour is, I don't know. I find the grenade chasing business especially baffling. What on earth is there in the game programming that makes them so interested in grenades in these areas, yet not in the valley? Is it actually some sort of 'Easter egg' somebody sneaked in? Seems doubtful; I'm guessing it's some unintended consequence of the Banshee being in an environment that wasn't planned for. If anyone reading this can shed some light on these behaviours or has a theory, I'd be happy to hear about it!
Having just played the first few levels of the original game this weekend, I also noticed some of the same Banshee behavior, as well as some other, related non-Banshee behavior.
I have a feeling that the scripting for Banshee pilots (or perhaps any AI unit using a vehicle like a Banshee, Wraith, or Shade turret) is "overlaid" on top of that unit's basic AI routines. So the normal Banshee behaviors we're used to seeing, such as the dive-bombing and so on, only exists in environmental areas where Bungie expected Banshees to be, and often times wrote specific scripts for specific areas.
Click 'read more' from the front page for the entire article.
When a Banshee strays outside that area, it's still an enemy AI in a vehicle; it doesn't drop dead or stop moving. But without the specific script, it seems like it doesn't quite know what it should be doing.
The first thing is the bashing behavior. Outside normal Banshee areas, Banshee pilots seem to forget about their weapons and just try to squash you. It may be that the information about the Banshee's weapons is in the Banshee-specific script. With that gone, the Elite behaves like one without a weapon (or perhaps one with a sword) and just tries to melee you. Since he's in a Banshee, the result is, he squashes you (or tries).
The grenade-chasing behavior Rockslider describes I've seen elsewhere. Covenant units in Shade turrets will do it. If you throw a grenade from an area where the unit in the turret can see the grenade, but doesn't see you, they will turn to track it. Unfortunately, they don't turn towards the direction the grenade was thrown from, which would be the sensible thing, but follow the grenade's flight to where it lands-- as Rockslider describes, like a dog chasing a stick. When a unit is on foot, following the flight of the grenade through the air is a prelude to diving out of the way if necessary (and sometimes even if not necessary or indeed prudent). A unit in a vehicle can't (or won't) do that, but the initial part of the behavior seems to remain.
What I found even more interesting is that sometimes it isn't necessary to "lure" Banshees into these situations at all. Or rather, your mere absence in a place where a Banshee expects to see you is enough incentive to go roaming. In Assault on the Control Room, during the "Rolling Thunder" chapter when you emerge from the underground tunnel into the area surrounding the large tower, there is a Banshee that appears. While playing this level I happened to be using a Warthog, and rather than attend to the units outside the tower first, I opted to drive the Warthog inside the right-hand door, clear out the interior, and then proceed outside. A lot of the grunts and jackals will follow you inside, and the Warthog makes easy work of the elites inside (stealth and otherwise) if you keep it moving.
When I emerged, I went after the Wraith and then took out the Hunters and Elites that appeared from the far exit to the area. It was then that I turned and looked back towards the tunnel and noticed a Banshee circling there. As I watched, it turned and flew down into the tunnel, and did not return.
I drove to the tunnel entrance, expecting to see the Banshee flying there. It was not. I drove all the way down the tunnel, and to the switchback where the loading zone is just before the door leading to the underground bridge. The Banshee pilot had wedged himself into a corner there. He took no mind of me as I tagged his ride with plasma grenades.
Rockslider also noted similar behavior in that same underground area:
I've also seen the weird behaviour in level 5. You know the 'platform Banshee' that attacks you in the area prior to the underground bridge? You can quite easily lead it down the tunnel into the bridge area. In the tunnel and bridge area it'll do squashing and grenade-chasing. When I led it over the bridge and beyond, it became zombie-like as I neared the enemies at the bottom of the rising mud tunnel. It just had its nose up against the wall, as if trying to move ahead regardless. Using the Warthog I was able to push it along so it was released up the mud tunnel. There, it continued its zombie-like behaviour all the way up, nose to the ground, occasionally needing a push to get it up over a rock. It got up into the next area but never did recover.
The zombie-like behavior seems to be associated with entering or crossing a loading zone; for whatever reason, mine got stuck in the first zone, before the underground bridge, whereas his didn't space out on him until the far side.