BUT: that you will KNOW when his second coming is here, as it will be like LIGHTNING in the sky! ...
... I wonder what a witness would say about this? because Jesus says that you will just KNOW it, as a sheer matter of FACT! because it will be just like lighting in the sky!
Here's the official WTS' understanding:
w74 12/15 p. 750 Who Will See "the Sign of the Son of Man"?
... his presence was to be like lightning, not in its being instantaneous and unexpected, but in its being seen over a wide area, in the open, for everyone to behold. (Luke 17:24; compare Psalm 97:4.) His followers would not keep their knowledge of his invisible presence secret, but would give it widespread proclamation. - Matt. 10:26, 27.
w89 8/15 p. 29 Do You Remember?
Why did Jesus say that his presence would be "as the lightning"? (Luke 17:24)
Jesus was here indicating that proof of his presence in Kingdom power would be seen over a wide area, enabling all who wish to observe it to be aware of the fact. - 6/15, page 8.
w93 5/1 p. 12 pars. 9-10 Shedding Light on Christ's Presence
Further, he would not conceal himself in some "inner chambers," his location being known to only a select few, that there, unobserved and undetected, he might conspire and draw up secret plans with accomplices for overturning the governments of the world and having himself anointed as the promised Messiah. No!
On the contrary, there would be nothing to hide about Jesus' having come as King, at the beginning of his royal presence. As Jesus foretold, in a global way, lightnings of Bible truth continue to flash over broad areas from eastern parts to western parts. Truly, as modern light bearers, Jehovah's Witnesses prove to be "a light of the nations, that [Jehovah's] salvation may come to be to the extremity of the earth."-Isaiah 49:6.
As you can see, particularly with the first and last quotes, the emphasis has switched from Jesus' lightning-like, evident-to-all Return to JWs' widespread preaching about Jesus' not-evident-to-all Return.