I have a one story, small house. I can hear everything that happens in our house. By about 9 months (?) my child was doing the kind of playing "alone" that you've described - where she would be playing in a space very near by, while I was cooking, making a phone call, drinking tea with a friend, cleaning, etc.
When our second child came into our home (2 3/4 when she started staying with us on and off), my daughter was approx. 20 months old. They *would* play for minutes-an hour on end. Not out of earshot, but out of line of sight. Then, when she started living with us, there was big transition and the kids fought All Day Long. *Sigh*. It was tough. I had to be The Negotiator all day, every day. Nearly lost my bananas. No kidding.
After six months of THAT, they went back to their delightful games. The imagination station.
Now, they are four and three and it's wooooooonderful. My go ride their bikes in the driveway (I live on a street that has cars pass MAYBE three-six times a day and slowly - and my kids KNOW to stay away from the road and within the tree line), and if the doors are open I can still hear them where ever they are. I'll check in every few minutes and ask them to make line of sight. I'll yell, "Check ins" and they'll come around the corner and say, "Here we are!" and then go back to play.
Inside, they'll play in their room, or in the living room...
If it goes silent, then they are up to mischief!
Good luck discovering what words for your family!