Personally, I think he knew the notepad would be photographed…pros use very high res and sharp lenses.
So the code is 'missing 15 - 19'…taken together with all the 'BIG' drops and Ides of March references of late, here what I think it means:
Drops hit on or just before the 15th. POTUS, Kansas, First Family, and other SS identified potential targets will be out-of-sight from the 15th to the 19th for safety and security.
Ergo, 'missing 15 to 19'.