Either one day Sherlock digs it out or it gets sent to their apartment but John has no idea what's in the video except that Sherlock is very protective over it - not allowing John to watch it or even tell him what is in the video.

Sherlock starts watching it when John is out or he watches it in his own room with little to no sound. Eventually John finds out what's on that tape and he becomes angry and frustrated because Sherlock just won't stop watching it and John thinks Sherlock is going mad.