It's said that the future perfect (will + have + past participle) is used to say that something will be completed by a particular time, as in
- By next summer I'll have finished my degree.
But in the example
- It's 5.30. Will Jay have left work already?
it seems to me that the second sentence could be replaced by
- Has Jay left work already?
which is of the present perfect tense, without any significant change in meaning.
Please let me know if I'm correct.