The store near me has always has at least 30 Macs queued up for repairs in the back-of-house Genius Room.
They don't have time to dick around with peoples Macs, other than to perform the designated repair, and do some quick testing afterwords.
They will ask you for your admin password in case they need to run updates, etc, which is something they'll usually do, even if your Mac is only in for a hardware issue.
They are not responsible for data loss, so you were smart to back your Mac up before you went in. If you're worried about your stored passwords, etc, use the Keychain Access program, right-click on the login keychain (that's where the bulk of your passwords are stored), and change the password to something different than your regular password.