Being newish to file sharing I have also had the misfortune to be confronted with horrendouse stuff like this and is quite terrifying. The thought that you have been logged somewhere against it is scary to say the least even accidentaly.
What can you do, who can you go to i have no idea.
As you dont know where it came from you dont know who to alert?
A very depressing state of affairs on a very depressing subject. Which the only solution (you may disagree) is not to file share which is the way I will go.
I am sure it will reduce eventually over time, till then i will give it a miss.
Mach1, a damaged person now.