There were two things Damien learned through his altercation with Wang Chen.
Firstly, even someone at the 4th class who had received a power boost from the Nox wasn't able to perceive the Void Essence he had quietly injected into Wang Tianhe's body.
This meant that although he still had to hide his Void Physique publicly, using its abilities in a veiled manner was still viable unless he was facing a genuine Demigod.
Secondly, if a 4th class appeared out of thin air, nobody would question it even if they couldn't sense the mana fluctuations, even the more powerful 4th class existences around.
Firstly, this meant reaching 4th class allowed some kind of great maneuverability or power that Damien didn't know about yet. Something that could allow all 4th classes to move unseen. Secondly, it meant he could summon his 4th class helpers from the Sanctuary without fear of revealing its existence.