I can't help but wonder

I'm sure I'm not the only person who has adopted a dog from a shelter and wondered what the dog's previous life was like.  
First off, I wonder why Iris' former owners didn't search for her.  They'd had her microchipped, so they could have checked with the animal shelter that picked her up off the street.  
Maybe the economy caught up with them and they decided they couldn't afford her.
I'm fairly sure Iris was a house dog, and allowed on furniture; she still has a lapse in judgement on that issue occasionally.  Whoever owned her didn't allow her to beg at the table; I know this because when we sit down to eat, Iris disappears.  
Like the dogs before her, Iris has learned to anticipate our morning walks.  She's on high alert when Cliff gets out of his chair.  If he's in the bathroom brushing his teeth and I'm at the computer, she runs back and forth between the two of us.  Dogs do love to walk with their humans.  
In Iris' case, it's more like her running through the woods at the same time we're walking, somewhere within a half-mile of her.  
Now that Iris has lived here for six months, I don't normally keep her on a leash.  Oh, there was a recent spurt of frog-chasing at the pond where she'd come back with muddy feet, legs and belly; while that was going on, I'd snap a leash on her for our walk in order to avoid the cleanup afterward.  She does fine with a leash, no pulling and jerking.  
But the frogs are now in hibernation and I let her run free.  
Here's what confounds me.  Once we're through the gate behind the house, she heads off to parts unknown; with her speed, she's quickly out of sight, and sometimes we don't see her again until the walk is over.  Other times she'll swing by and check on us once or twice, then head off again.  
She's free to do all this running any time she's outside, but she only does it when we're taking our walk.  Since she never heads toward the road, I don't pay a lot of attention to where she goes these days.  Why doesn't she go off on a crazy chase while we're inside?  She certainly isn't worried about our companionship, or she'd stay with us when we're walking. 
How does she feel, I wonder, running as fast and far as she wants, after spending her previous life as a house dog?  From her expression, it must be glorious.  The whole world is hers.  
Cesar Milan makes fun of people who wonder about their dog's past life.  "Dogs live in the present," he says.   
He's probably right, but I have fun delving into my dog's psyche, wondering about her past; it's as good a pastime as any.