Well, I've just had a conversation with one of the technicians at Abbey Road, and he wasn't aware that they had any 7520's, but, as he he said, it's a big place with many people doing different things so it's possible that there are some there. However, he did say that they used to have a large number of 7506's, but that gradually over the years they had 'migrated' out of the building, courtesy of people who thought that they would be better employed elsewhere(!).
They used the 7506's mostly for the conductor of orchestral sessions when the the conductor needed a high quality headphone monitor. They use DT100's for tracking and engineers generally use Sennheisers, although he couldn't be specific about which ones.
I suspect that the 7520's would be too expensive to consider for tracking use, and of course production is always done with open headphones or monitors, so it was probably just a rumour I'm afraid.............