Ty pewnie myślisz o przymiotniku "secret" a nie rzeczowniku.
secret jako rzeczownik jest C - countable:
secret noun
1 [C] a piece of information that is only known by one person or a few people and should not be told to others:
Why did you have to go and tell Bob about my illness? You just can't keep a secret, can you?
A close couple should have no secrets from each other.
Aren't you going to let me in on (= tell me) the secret?
There's no secret (= everyone knows) about his homosexuality.
She makes no secret of (= makes very clear) her dislike of her father.
That restaurant is one of the best-kept secrets in London