If you know that 1 of 4 individuals is an apostate because only that group has access to documents that have been leaked, then you can print out 4 nearly identical documents, each with only 1 difference from the others, (A different word with the same meaning or a phrase worded differently,) and give each individual a different document than the other 3 so that no two of them have the same document, keeping track of which individual has which document.
Someone's been reading Tom Clancy. That's Jack Ryan's Canary Trap :D