Within the Prime Minister's Office, the PCO is the traffic cop, so to speak, of documents that would go to the Prime Minister. These tracking sheets are actually very extensive, from what I remember. They not only indicate that they go to the Prime Minister, but indicate every official they go to on the bureaucratic side, depending on the department. They also track, obviously, which agencies they would go to and what political staff they would go to.
These would be available, and it would be very simple for the Prime Minister to direct the PCO to make public that tracking document to find out where this mysterious briefing note went and who it was delivered to. Tracking notes, from what I gather and from what I remember, are not top secret or classified documents.
If the Prime Minister and the government were truly open to figuring out where this tracking note somehow got lost in delivery, it would be very easy to find out which officials, which security agencies and which political staff received it.