>As for them finding out, I don't know where you are at in the world, but in the US their is no way for them to find out unless you provide them with that information.
Professional networking tends to be a big factor when moving from job to job (this may be extremely different in startup communities). This tends to mean that as your career progresses, you have probably worked with 1 or 2 people at a prospective employer. Those people are generally asked about the perspective hire, including their estimates of what that person has made.