Knowing that calling for her physically would just cause disaster, Bruno avoided doing that. However, he still needed to warn her about the incoming danger, which he did by creating a scene that would make her aware of an incoming threat.
He absolutely avoided the creation of a scene that would move Joia's focus towards him instead of an incoming threat…
Bruno's plan is actually quite simple. He made the indication that he is dealing with a threat, which made Joia ready for a threat that might come her way as well.
As to how he did it… He used his automatic rifle to shoot the shadow that was still faraway, which caused the ground and his body to tremble due to the knockback and the power that comes after the shots. Because of the trembling, Joia was able to connect the dots, and figured out that Bruno was already fighting something. Knowing that he was already fighting something, she became ready to fight as well…
