The circling behavior is, as I read, almost always them waiting for a warm current to take them elsewhere.
They're not very vocal and are very social, so circling over carcasses might be a way to signal to others that they've found something.
They also form kettles as a big communal thing, too, for neither of those purposes. I've seen as many as 50 or 60 circling together for well over an hour, with no food nearby and no clearly no interest in going anywhere else.
(This is all turkey vultures in the US. These African vultures might have different behaviors, but I wouldn't be surprised if they are mostly comparable in terms of variety and sophistication)
George the turkey vulture: It's a nice day to hang out circling.
Rambo the turkey vulture: Hey George, let's hang.
Pappias the turkey vulture: Where's the food? No food. LOL. You guys!
All: LOL (but silently).
Sydney the turkey vulture: Where's the food? No food. LOL. You guys!
All: LOL (but silently).
Violet the turkey vulture: Where's the food? No food. LOL. You guys!
All: LOL (but silently).
Sydney the turkey vulture: Where's the food? No food. LOL. You guys!
All: LOL (but silently).
Seven the turkey vulture: Where's the food? No food. LOL. You guys!
All: LOL (but silently).
Arnold the turkey vulture: Where's the food? No food. LOL. You guys!
All: LOL (but silently).
Angel the turkey vulture: Where's the food? No food. LOL. You guys!
All: LOL (but silently).
...
60 vultures later someone observes 60 vultures circling and concludes:
A. There's definitely no carcass, as that's a myth.
B. There's definitely a carcass, as there's 60 vultures.
C. I'm going to find some water to drink.
And yeah, on close read, you'll see that they're all trying to communicate that you can't assume that the vultures are circling a carcass whenever you see them circling because they have other behaviors that involve circling as well. But it's an easy detail to miss in some of them for sure!
I’ve read elsewhere vultures are actually good signals of gas leaks for this reason.
They don’t have much of a reason to circle a carcass they intend to eat unless perhaps it’s a lagging indicator and they’re leaving the carcass. They don’t circle dying things because they don’t have the means to detect dying things.
I’m not convinced circling vultures is a good sign that there’s a carcass below them.