When you get attacked by the cultists, which initiates the Dragonborn DLC, but don't find the note on any of them, what then? I got attacked outside Breezehome and while I took care of one cultist Lydia and the guards took care of the others. I never saw how many there were but I only found two bodies and now I have a quest marker in Whiterun that I can't locate the target for =(

Will you get attacked again with the chance of finding another note?

Maybe he fell through the floor? It's a fairly common, if unfortunate, bug. Best solution is just to reload and try again. Alternatively, if you're on PC, try checking the UESP page for the quest stages and use the console to force yourself to the next stage.

Sounds likely. I hate forcing quests. I hope they come back and try again. Can you just proceed without finding the note?