Who Is Watching My Lessons? 👻

Half serious, half a joke question... 

I have been a citizen since a few months here on CG Cookie and finished a few courses. But it has happened to me several times while browsing the courses: I open a course I've never opened before, and I see that some random lectures in the middle of that course have green tick marks, as if though I have watched them. But I haven't. 

I haven't shared my login details with anyone, I don't share my devices, and moreover no one else in my house does Blender 3d...

So is it just some system bug? Or else who is watching my lessons? 😄👻