Anonymous visitor xDB data is stored according to the cookie guid that was placed on the visitors computer as
SC_ANALYTICS_GLOBLE_COOKIE. As long as the user doesn't clear his cookies, subsequent visits/interactions will be tracked to that anonymous visitor.
In the event that the user does self identify in some capacity, running
Tracker.Current.Session.Identify (identifier) will take the current anonymous visitor record and merge it into a Known visitor record that can be recalled using the Identify method.
At any point cookies are cleared before the Identify method can be executed, the visitor will recieve a new guid, and thus the old interactions tracked by the old cookie guid will no longer be associated to the new visitor guid. This means that those old interactions will not be merged to the known visitor contact when
Identify () is ran. While these interactions are still in xDB and can be used for analytics, they are lost to being able to be associated to a known contact.