Dog's rep tells TMZ, some of the tipsters offer vague clues -- something he's used over the years to eventually develop solid leads -- and he's also been getting a slew of emails from people who just want him to jump in because of his track record in finding missing persons.

Here's the problem. When cops were setting their sights on Brian Laundrie, the man who murdered Gabby, Dog was honeymooning near the Florida home where Brian's sister lived, and he decided to jump in and help. The video of Dog knocking at the sister's door made the rounds and was heavily criticized ... some claiming he was meddling in a police investigation.

It's kind of a case, Dog thinks, of no good deed going unpunished. His rep says the way he operates ... he will not get involved in a missing person's investigation unless one of 2 things happen -- either he's invited by a family member or law enforcement to get involved, or 21 days have passed since the person was declared missing.

The reason for 21 days ... Dog says that's when missing persons cases typically go cold.

So, Dog is now waiting ... waiting for an invite or for 3 weeks to pass. He's also waiting for solid clues on where Kiely might be.