Soooo forgive me if this has been suggested before, but:
We've seen photos of a spotlight on the roof of the stadium. And then another photo emerged of another spotlight next to it. And Heatherwick himself said it wasn't just going to be a 'thing', it was going to be an event. What if those spotlights were for people. And instead of two, there's a spotlight behing EACH pyramid on the roof. For somebody with a torch to light a flame on top of each pyramid section. In in an instant, the cauldron has is revealed to have been right infront of our very own eyes.
Is there a rule saying there must only be one cauldron? Perhaps the plan is to have flames burning atop the stadium from each pyramid for the entirety of the games.
It's a bit of a mental suggestion...but there ya go.