Leo only noticed that there was a monster in the cave because of his Wall-Hack that was activated all the time. If the Wall-Hack was off, he'd probably be like the other kids, completely ignorant and relaxed thinking he was safe.
Then calmly, Leo raised his hand to his mouth in a sign of silence, startling the other boys, but they soon went quiet.
Confused, the boys soon became alert and looked at Leo to understand why he was acting like this.
Leo pointed to the back of the cave with one hand and with the other hand he made the 5 sign.
Dave was the quickest thinker and asked through signs if Leo was saying there were 5 monsters or a Bronze 5 Stars monster.
Seeing Dave's question, Leo only showed one finger, indicating that it was just a Bronze 5 Stars monster.