What a username search checks
A username search takes one handle and finds out where it is registered. This one covers 687 platforms in a single pass: social networks, forums, developer and gaming sites, creator platforms and the long tail of communities that most people forget they joined. Each live profile comes back as a card with the picture, display name and public details the platform shows.
The same handle is searched in breach records and in logs taken from infected computers at the same time. A username used to sign in somewhere is often a username that has leaked somewhere, and a match there shows the service or the site it was saved for.
The same handle is not always the same person
A short or common handle is taken on hundreds of sites by hundreds of different people. A match proves an account exists under that name, not who owns it. Before treating two profiles as one person, look for something that ties them together: a link from one profile to the other, the same picture, the same bio wording, or a join date and activity that fit the same life.
Unusual handles are far more telling than common ones. A handle with a birth year, an in-joke or an odd spelling in it is usually one person everywhere it appears.
Searching your own old handles
Most people have used three or four handles over the years, and the old ones are where the forgotten accounts are. Search each of them, including the ones from school, then close what you no longer use. An abandoned account with a reused password is the easiest way into the accounts you still care about. The guide to checking whether your information has been leaked covers what to do with each kind of match.
What a search will not find
Deleted accounts, accounts that hide from logged-out visitors, and sites outside the list do not show. Neither do other spellings: a search is for the handle exactly as typed, so run the variants with a dot, an underscore or a number at the end as searches of their own.