GitHub distributes a simplified Git client for Windows they call GitHub for Windows; it authenticates via GitHub account, and although it usually uses HTTPS remotes for local repositories it creates, if you tell it to use an existing local repository that has an SSH remote on GitHub, it will use your GitHub credentials to set up an SSH key on the GitHub account for itself. But where does it keep the SSH key in the filesystem?
%HOMEDRIVE%%HOMEPATH%\.ssh\id_rsa.pub
is where the key is.