Place it at the reveal in an edit, right after the shot that introduces the strange object and just before the next shot shows the reaction. That position lets the question mark the discovery without carrying the whole scene, and the following shot can answer it visually.
About this sound
This clip repeats the Vietnamese question 'cái gì kìa' twice in just under five seconds, presenting the same short phrase back to back with no other words. The repetition makes the question feel curious and slightly puzzled, as if the speaker has just noticed something unexpected and wants someone else to look too.