Darn those digital files - they are so easy to share. There are a few websites that encourage students to post documents in order to gain access to other’s documents. These documents can be your syllabus, their notes, or your exam keys.

SEARCH
The sites tend to turn a blind eye to what is posted, but are very responsive to requests to take things down. In your spare time, you could search for your class / your name / your material and see if there is anything egregious out there.
Here are two to start with:
CourseHero                 
OneClass       

Quizlet is a site where students create and share flashcards. This tends to be more honest, but I have also found decks of flashcards that are actual exam questions/answers.

REPORT
Here is how to report copyright infringement if you find things you want removed:
CourseHero (don’t be afraid of the threatening legal wording - it seems like bullying to me)
OneClass  (submit a ticket and respond to the email)
Quizlet

Want to do more than take it down? This quarter, one of the students on CourseHero had used her full name as her username to post some of my material. I turned her into our campus OSSJA and they are investigating it. (I clearly state that students are not to share my materials and have them acknowledge academic honesty on their homework every week.)


ps. Here are two others that you don’t need to worry about.
GradeBuddy doesn’t seem to be used by our students.
Koofers is obnoxious to get into and mostly outdated. I have submitted a blanket request to remove exams from current faculty in our department.