Yes but links can leak in the most accidental fashions. Google seems to find a lot of stuff that makes me wonder how Googlebot ever found such links. Part of the explanation might be referer logs, and such.

Anyway, the point is, it's smarter to be explicit and also opt-out using robots.txt or something similar.

If your side gate was open when the googlecar drove by and put it on streetview, does that mean that anyone else is entitled to wander in and take things should the gate be open as they pass by?