That's something they seem to do, for sure. It's just not the only thing.

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)