Rio had already sneaked inside the base a while ago with the help of his new 'friend' (forcefully made); he then made himself invisible using light elemental's properties.
Once the S-rankers were off-guard, he went and disarmed the entire base. Not only did he break their communication methods with the outside world, but he even managed to set up space-jamming artifacts everywhere.
His sword intent and assassin skills are a perfect match, and he can even hide the spatial fluctuations of a space-jamming artifact for a while.
If it was someone like Alverto or Link, they could have easily noticed the spatial fluctuations, but it was not that simple for ordinary S-rankers.
Rio then killed the orc warriors who were in charge of communicating with the 'Orc King' and 'erased' their bodies from the face of the earth without leaving a single clue.
