I think this must be the film footage that Mick talks about in his most recent autobiography. Similar to the Eagles, they hired a film crew to shoot a concert on their 1977 tour and then forgot about it for years and years.

At the time of the book, about 5 years ago, IIRC Mick was looking to put it together as an officially-released DVD. I guess that didn't happen, and the film footage just... got out there.