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.
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: