I've observed it twice at Heathrow.
First time was about 30 years ago; Heathrow switched from westerly to easterly; on radar we watched the stream of traffic positioning downwind for easterly landing, then an aircraft appeared westerly off Heathrow and every aircraft in the inbound pattern from both north stacks and south stacks (about 14 or more in total) did an orbit. No danger.
The second time was a few weeks ago. I was in my back garden and aware of MID departures from runway 27s passing overhead, then noticed aircraft positioning downwind for 09s; just after the first ones passed, another 27 MID departure passed overhead.
No danger that I could see, but I don't have access to radar any more so I couldn't see their altitudes.