I have read about the “Opt-Out” plugin for Google Analytics. This allows end-users to indicate that they do not want Google Analytics to track them. Read more here:
https://chrome.google.com/webstore/detail/fllaojicojecljbmefodhfapmkghcbnh?hl=en
My question is: could this mean that if my visitors have this plugin installed, that I will have visits that do not at all show up in Google Analytics?
Yes, this plugin allows user to not be tracked in Google Analytics.
There are other multiple extensions that disable analytics tracking. Since all tracking is done client side there’s always the possibility to disable it.