When you have a long list of attendees in a large organization, it's almost impossible to do that with everyone.
Pinpointing the source isn't necessary, though, only knowing that the leak occurred and the approximate number of people "in on it". Even limiting this to leaks to the media for information shared at company meetings (so the number of people is equal to the number of employees) could provide interesting data, assuming a large enough sample size (and that leaks are numerous enough).
Mole-Hunter-As-A-Service™
"Three may keep a secret if two of them are dead." -- Benjamin Franklin