First rule of enforcement (whether law or regulatory): write down everything. Enforcement bodies teach notetaking as a mandatory course for a reason. You never know what tiny, seemingly insignificant and easily forgotten detail may prove pivotal if it goes to court.
So not only will Comey have notes, they will hold a higher legal weight because they will be taken at the time of or just after the incident(s). They will be dated and detailed enough to serve as evidence, simply because that habit has been drilled into him over the years. Some enforcement/regulators have notes on every single phone call and meeting they've had over the years. Not necessarily because they think that there is something nefarious happening (though in this case Comey might have) but simply because it's habit and you never know what might be a make or break detail.
Edited to change the word "criminal" which has a narrow and specific meaning, replaced with "pivotal" which better reflects the point.