The person hiding in the mirror was Illuso from the Hitman Team. Fugo ends up being dragged into the mirror by Illuso's Stand, Mirrorman. Fugo then summons his own Stand, Purple Smoke. Despite Giorno wanting to find Fugo, Abbacchio decides to go look for the key, since that was their mission. However, once Abbacchio finds the key, he also gets dragged into the mirror world...
That was the sound of the key 
falling to the ground.
كان ذلك صوت سقوط المفتاح في الأرض
Das war das Geräusch des
zu Boden gefallenen Schlüssels.
Ese fue el sonido de la llave
contra el suelo.
Ese fue el sonido de la llave
contra el suelo.
Ça, c’était le bruit
de la clé tombée sur le sol.
Quello era il rumore della chiave
che cadeva per terra.
Esse era o som da chave
caindo no chão.
За углом звякнул упавший на землю ключ.