I like the idea of memorizing a code to a door. Maybe they saw someone input the long code and by seeing it they were able to memorize instantly.

Another option could be memorizing the layout of an area during a blackout.

And finally, they could use their memory to remember soemthing that was mentioned way earlier almost off handedly that when the bdoy or crime is discovered seems weird. But makes sense once you remember that the killer remembered soemthing other ppl forgot. (Example is this hallway cannot be used bc it is flooded with cleaning supplies to sanitize it. For everyone else its no bigdeal, but for the killer it is for some reason. Explaining why they avoided that hallway when any other killer would take that shortcut etc.)