According to law enforcement sources ... Justin was shopping at a Best Buy in the San Fernando Valley when the adult male -- not a crazed tweener fan -- approached him and took the photo.
We're told the man claims someone in Justin's entourage -- someone he thinks is a bodyguard -- immediately, and angrily, told him to delete the photo ... which he did.
The photo opportunist -- who we're told is not a paparazzo -- felt threatened enough after the encounter that he called police ... but when LAPD officers arrived Justin and his posse were long gone.
We're told police took a report from the alleged victim, and plan to investigate his claim.
Justin's camp had no comment.