Mom noticed it first. We had a Labrador Retriever and he would not be satisfied until he had checked you out, and he was pretty insistent about it...
He wanted to smell your breath. He'd come up close to you and stick his nose in your direction. We learned to exhale toward him and watch him inhale and sample the smells there, and then he'd walk away satisfied.
It's been my experience that most dogs will do this...
When I come home from work and sit in my recliner, Lucy and Yogi will come stand on my chest and look at me in expectation until I gently exhale in their direction. Then, like they've completed an important task, they'll relax and curl up in my lap.
Experts say most dog's noses are 10,000 times as sensitive as ours, and bloodhounds even more sensitive than that. We know dogs can sniff out cancer, can sense an oncoming seizure in someone with epilepsy, can smell a cadaver under 30 feet of water.
I tried to do a little research on the subject and found this comment. That answer really tells me nothing, but it verifies that others are experiencing what Mom first noticed so many years ago.
I want someone to learn to speak with dogs so they can tell us how much they learn by smelling our smells.
I bet it'd be spooky what they could tell us.