“As he sat on the Mount of Olives, the disciples came to him privately, saying, “Tell us, when will these things be, and what will be the sign of your coming and of the close of the age? (27) For as the lightning comes from the east and shines as far as the west, so will be the coming of the Son of Man. (28) Wherever the corpse is, there the vultures will gather. (29) “Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken. (30) Then will appear in heaven the sign of the Son of Man, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory. (31) And he will send out his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of heaven to the other.” (Matt 24:3, 27-31)

I have read scores of commentaries on Matthew 24, and I am always baffled that there are so many different interpretations of this question (preterists have the most strained readings). The most natural reading is that the sign is the lightning, which represents Christ’s Shekinah glory — his divine presence-glory.
“Parousia” (Coming) means presence. Ezekiel sadly witnessed the inverse of the sign of God’s presence departing from the Temple.

“And the glory of the LORD went up from the cherub to the threshold of the house, and the house was filled with the cloud, and the court was filled with the brightness of the glory of the LORD. Then the glory of the LORD went out from the threshold of the house, and stood over the cherubim. And the glory of the LORD went up from the midst of the city and stood on the mountain that is on the east side of the city.” (Ezek 10:4, 18; 11:23)