I highly recommend The Dog Bible...the Mehus-Roe one....great book...explains this pretty well.
When we adopted our adult dog almost 5 years ago, she would do this. Of course it was a little shocking to us, and I worried about her picking up some sort of bug. We would tell her NO, and she stopped doing it.
She had been in 2 shelters, and was very, very thin when we got her. In fact I think the weight she did have came from being regularly fed in the shelters. We were told she was an owner surrender, but we believe strongly that she was either neglected, or, being a submissive, never got her share when the owners fed all the dogs at the same time.
Well now she's a pampered pooch and an only child. We feed her only Wellness canned food and treats, and although she's a mixed breed with an unusual body type, and it's hard to tell for sure, we and our vet think she's around the right weight. She shows no interest in that behavior anymore, although if left on her own...who knows? I've heard that scent hounds tend to do this sometimes even when they are well fed....I guess it just smells yummy to them.
Your dog just might be hungry...maybe offer a few treats during the day, or, with a Vets supervision, increase the food. She may have to take supplements too...call your Vet.