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.