— used to say that you have little or no knowledge of something说不定；不知道
For all I know, he left last night. [=I don't know when he left; it's possible that he left last night]说不定他昨晚就走了。
She may have already accepted another job, for all we know. [=we don't know what she has done; it's possible that she has already accepted another job]我们不清楚，也许她已经接受了另一份工作。