Is "Kuchh aaya hai" correct?

Why

Sameer picked the right general idea — something unknown or uncertain — but Hindi splits 'unknown' into two words: one for people, one for things. 'कोई' is for an unknown or uncertain PERSON ('कोई आया है' — someone has come), while 'कुछ' is for an unknown or uncertain THING ('कुछ गिरा है', 'कुछ खाना है'). A knock at the door means a person is standing there, not an object, so 'कोई' is correct — 'कुछ आया है' sounds as if some object walked in by itself. Test it: 'कोई बोल रहा है' (a person) versus 'कुछ बोल रहा है' — the second sounds odd because speaking is something people do, not things. His sister is right: 'कोई' for people, 'कुछ' for things — a knock calls for 'कोई आया है'.