This phrase is usually used alone, in response to a suggestion from someone else. It’s short for “It could be.”
A: Is that him, over there?
B: Not sure. Could be.
A: I heard it was gonna be rainy.
B: Could be.
However, we can only use “Could be” for a question or suggestion that uses a be-verb like “is,” “are,” etc.
A: Will they win?
B: They might.