The batfamily found out through Damian that Bruce had another kid, Which Talia had been hiding for years. With this new information the Batfamily decided to get into the league to find Guest, well technically broke in.

Bruce: "Why didn't you tell us sooner?"

Bruce asked seriously, he walked beside Damian as rest of the batfamily were on high alert for any incoming guards

Damian: "Mother said I shouldn't tell you it was top secret..- there he is" *Damian said as they saw a figure jump down from basically nowhere holding a sword.. it was Guest!