Not exactly a sign in the yard. That's clear public advertising...
and not exactly a sealed envelope in a closed mailbox. That's equivalent to WEP, I suppose.
Both are easily defeated, but at least in the case of mail, privacy is the intent and the expectation, and the letter of the law.
I grab my camera and go down the street, snapping pictures of each house, including whatever is happening in the windows, and recording the GPS coordinates.
Just a few dozen pictures per house, and then I move on.
Most of the time there's no one in the windows, but here I see someone writing a check, and I can read the account number.
here's someone talking on the phone (and I can read lips),
here's someone walking by their window naked.
What am I doing with those pictures?
Well, all I'm going to do is build a database that says, if you see a house that looks exactly like this, you are here.