Screams
Necromancer Hecates Ruins were located in the middle of this forest without a dense population of humans for several thousand miles. In addition to this, it wasn't out in the open either, it was hidden within an illusion array that required very precise steps to enter.
Unlike the array Ryu broke to enter the Natural Order Sect, this one was meant to be opened like a code. Meaning, as long as you didn't make a mistake, it was no different from a leisurely stroll. Although, if others saw you taking this 'stroll', they'd believe you to be a lunatic walking in circles. This was yet another reason why Ryu found it baffling that three adventure squads could just 'stumble' upon this ruin.
Of course, Hecate and those who built Ruins prior to their deaths like her didn't leave the discovery of their Ruins completely up to chance. What would be the point in spending so much time and effort just to build a Ruin no one would ever be able to enter?