If all viruses were detectable with antivirus then malware would be easily found and no more hacks would happen.
Some viruses are hard to detect on purpose. They evade antivirus software, system administrators and other software.
But a scan of a system will show a port open which requires further analysis. Because the virus or malware wants to phone home, well it has to have a port open to do that.
Now we can find it: with Sigma