Someone with physical access to your computer could use software to discover passwords to various services and log into your accounts. But they might not have to go to the trouble. Another way cheaters can unknowingly give away their activities is by letting the computer, software applications or various Web sites save their usernames and passwords so that they don't have to type in their login information. Or they might even have programs like instant messaging (IM), e-mail, or anything used regularly set to start up and log them in automatically anytime the computer is booted. The computer itself might even be set to automatically log in without a password (which is moot if you share a computer anyway).
Some things, such as some IM clients, will default to opening at startup if you do not change the settings during or after installation. Your computer, browsers or other software can even be set to save passwords for applications and Web sites for you. Many Web sites allow you to select "Remember Me" to save your information via cookies so that you don't have to rekey your information every time you visit them. And password management software is readily available to help you keep track of multiple logins or to log you into things with only a single master password, for those of us who just don't have the facility to remember a gazillion usernames and passwords.
These services can be convenient time-savers, but can also let others who have access to your computer easily log into your accounts to do things such as view financial transactions or read your e-mails and IMs. If you've been using your computer for illicit goings-on, they could make it easier for someone else to discover the tell-tale signs.