I feel like most parents are generally concerned about knowing where their child is at all times. I definitely like to know the vicinity in which Patrick and Lilly are playing, but what I find more important is knowing that they know my location.
If we get separated, I have a lot more tools at my disposal to find them, then they do to find me. Being more than three feet tall, for instance, and not consistently walking into people (cough-Lilly-cough).
We went to the children's museum today, and I generally let them have the run of the place - though they always stick in one area at a time, and we all move on together (mutual unspoken agreement, I guess). When we got to a new area (the balls, the kids' town, the water), I would pick a spot to sit down, and verbalize to them, "I'm going to sit right here." They may then get out of my sight, but they know where to find me.
I do the same thing at a playground. We all walk over to a table or bench or something remotely comfortable, I set my things down, tell the kids that's where I'll be, and then they run off and play. It lets me not freak out when I can't see them. It gives them assurance that they can get to me whenever they need to.
Think about it: adults seem to always know where kids are, but the kids don't necessarily have that same assurance. I'd hate to be a kid and have to hunt around, even a little bit. My two just backtrack to where they left me, and I'm sitting right there, doing a crossword puzzle or something.
It lets me feel secure about them, and relax in peace. Win!