As we reported earlier this week, on Tuesday, authorities executed a felony search warrant and raided Bieber's house, arresting Lil Za for possible drug possession in the process (as the drugs were in plain sight). While looking for evidence linking Bieber to the crime, they also took his cell phone -- reportedly, right out of his hands.
While the authorities are searching his cell phone primarily for evidence related to the "egging incident," perhaps hoping to find text messages or emails bragging about it to friends, Bieber is apparently more concerned that they will find discussions about drug use and the like.
That wouldn't be prosecutable, because there is no physical evidence. Bieber, though, is afraid any such evidence will leak out. Considering the state of technology and the Internet, the worry is probably founded.
In addition, he is also concerned about naked photos on his phone. It's unclear if they are selfies, or shots of someone else, or even shots or videos of possible "action."