Law enforcement sources tell TMZ ... Maddie's family filed a police report with LAPD, telling cops someone hacked Maddie's phone back in September. The hacker's been hitting up 15-year-old Maddie, saying he or she's gotten a hold of pics and messages.

We're told LAPD's investigating, but here's what's a bit bizarre -- sources say the hacker hasn't demanded money and neither the pics nor messages have been released ... at least so far. It's unclear what the hacker wants.