Someone, something, somewhere: ai đó, gì đó, đâu đó

A question word with đó after it means "some-": ai đó (someone), cái gì đó (something), đâu đó (somewhere), khi nào đó or lúc nào đó (some time). Use them in statements, for a person, thing or place you can't or don't need to name: Hình như có ai đó ở ngoài cửa. I think there's someone at the door.

They sit where the person or thing would go: Em muốn ăn cái gì đó. I want to eat something. Cuối tuần mình đi đâu đó chơi đi. Let's go somewhere this weekend. After a noun, nào đó does the same: một ngày nào đó, one day, some day.

In Hà Nội you'll hear đấy as well as đó: ai đấy, cái gì đấy.

Compare the question you know. Có ai gọi em không? asks whether anyone called; Có ai đó gọi em says that someone did. And said as a question, Ai đó? means "Who's there?", because đó also means "that". It's what you call out when someone knocks.