Lonesome Grunts in the valley
Associated movies
I have to give special mention to something quite odd that you can encounter in this level. In two separate places on Heroic, a dropship drops a single Grunt into the valley linking the three survivor areas. A single Grunt? Um, I think you'd have to question the tactical astuteness of Covenant high-command there. I mean, unless he's some sort of turbo-charged Special Ops super-Grunt, which he isn't, you might just as well drop off a sack of potatoes or something, because it would be about as effective. Possibly more effective actually, as you might at least slip on a loose potato and sprain your ankle. Still, I like a bit of grunty fun, and these guys are just the ticket.
One gets dropped at the first entrance to the rockslide area when you've dealt with the cliffside survivor area where the survivors hide underground. The other gets dropped by the stream at the right entrance to the cliffside survivor area when you've dealt with the hillside survivor area where the survivors hide at the top of a hill. In both cases, you're likely to see the dropship still hovering there. It leaves when you look its way or when the dropped covie takes damage, or when you get sufficiently near at ground level.
However, in each place you sometimes get an Elite instead of a Grunt, whilst on Normal or Easy you instead seem to get a group of three Grunts, or two Grunts and an Elite. On Legendary you don't seem to get the drops at all, which is a pity.
Grunty fun
The Grunts are great for observing with the sniper scope, and you'll probably see them doing that odd jumping up and down behaviour. Looks like a cross between callisthenics and being excessively bored. It's also easy to creep up close behind and examine them really closely. The one by the stream offers an extra bit of fun, namely seeing him run around in the shallow stream, making little grunty splashes as he goes; surely the only time you ever see a splashing Grunt. Gotta say, he's one cute little critter, and I had a lot of fun making him chase around. But business is business. Now, where did I put those grenades?
UPDATE: the two ships feature briefly at the end of BCM376.