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...
Snakes can tell where humans are 
by sensing their body temperature.
تستطيع الأفاعي معرِفة أماكن البشر
من خِلال الشُّعور بدرجة حرارة أجسامهم
Schlangen können anhand der Körperwärme
bestimmen, wo sich ein Mensch aufhält.
Las serpientes pueden detectar
a los humanos por su calor corporal.
Las serpientes pueden detectar
a los humanos por su calor corporal.
Et ces petites bêtes,
I serpenti riescono a individuare gli esseri umani
dalla loro temperatura corporea.
Cobras conseguem detectar humanos
pela temperatura corporal deles.
Видишь ли, змеи могут 
найти человека по теплу его тела.