If you didn’t know, a far better way to monitor what’s happening with your Steam account than chat logs is to go to your Account Details–>Security & Devices, and you can see who’s accessed your account, from which location, and from which device. You can hit the “Sign out everywhere” button, and then no one should be able to get into your Steam account without access to Steam Guard on your own personal phone that you probably carry on your person at all times. You don’t necessarily have to shut your computer off when you’re not on it, but it’s good security practice to at least lock it (Windows key + L) when you step away. Even then, the only people who could access it if you’re not doing that are people who share the same physical space as you, like your family or roommates.