An anonymous user is the account that Perceptive Enterprise Search will use when it cannot identify the calling user. The anonymous user is a standard Windows account that Perceptive Enterprise Search creates on installation. The account is local to the web server (it is not a domain account).
As it is a local account, it does not have permission to access network resources; the searching website will not be able to find any documents that reside on the network.
You can overcome this by disabling the anonymous user which results in each user having to provide login details when they access the website. To disable the anonymous user: