Choose what the concealed identity changes
An undisclosed profession changes expectations differently from a secret family background or a supernatural nature. Look for who knows the truth and why the character keeps it private. The premise becomes more specific when you can identify what would be lost, gained, or put at risk by a revelation.
Is hidden identity the same as mistaken identity?
Not necessarily. Concealment can be deliberate, while a mistaken identity can begin with an assumption no one intended to create. Neither guarantees that the audience is kept in the dark. A story may reveal the truth early and build tension around when other characters will learn it.