I was playing TF2 at a friend's house and I believe I forgot to log off. Does deauthorizing all computers with Steam Guard log me out, too? Or do I need to do something else?
You can now log out on all other computers by clicking the "Deauthorize all other devices" button at https://store.steampowered.com/twofactor/manage.
All you need to do is log on from another location. All other steam sessions will be automatically logged off.
If you saved your credentials on this other computer, then that is another story. I would de-authorize that other PC and also change my password.