However, lets say that a month from now a new Linux exploit is identified on a different port. As an alternative, script kiddies share or even buy databases of vulnerable systems from each other. The script kiddie can then exploit your system without even scanning it. Just because your systems have not been scanned recently does not mean you are secure. The more sophisticated black-hats implement trojans and backdoors once they compromise a system.
Backdoors allow easy and unnoticed access to the system whenever the user wants.
The trojans make the intruder undetectable. He would not show up in any of the logs, systems processes, or file structure. He builds a comfortable and safe home where he can blatantly scan the Internet. These attacks are not limited to a certain time of the day. Many admins search their log entries for probes that happen late at night, believing this is when black-hats attack.