Ok you said laptop so I'm guessing its using wireless right??
Its also possible that someone was war driving around your house and maybe cracked your wifi password and got direct IP level access to your ubuntu laptop over your network (this would be an easier exploit then over then internet) and from there found an unpatched application and exploited it?
Was your ubuntu laptop all up to data??
Also you could try check /var/logs/ and check the logs for various services and maybe find out what they did. Also check your router firewall log if you have one.