Broken Condom On The Floor !

Bra

okay so i left my bf home alone for a few days , i just got home and was cleaning my bathroom last night and i found a piece of a broken condom on the floor! we dont use condoms so i know its not from us. He swears that the toilet backed up and it must have come from there. But if we dont use condoms then why is it in our toilet ?? we live in a apartment complex so it it possible that other peoples waste came up too?