[quote name='catapult37']That seems logical to me. But I sometimes hear people say things like "please don't come in and cut down all my trees"... does that just mean person A left their DS unattended, but open and connected? Otherwise I think you'd always know when someone was in your town.
Or is it just that it's possible to be playing and have someone visit another part of your town offscreen without you knowing?[/QUOTE]
You'll know when guests come into your town because it announces it (though I've missed that announcement before because I was off doing something). Once they're in there, though, they can be off doing whatever and unless you're following them around, there's no way to tell what they're up to (though you can chat with each other no matter where each of you is in the town). Hide and seek can be fun sometimes, especially when you allow hiding in buildings.
Also, some people leave their gates open for long periods of unsupervised time for the benefit of those who want to come sell turnips, pick up an item, or whatever. Since the only people who can come in are those whose friend codes you've registered, the danger is minimal. But every so often you'll add an asshole who seems nice at first but then makes trouble.