The comparison of the parousia to the Flood of Noah emphasizes the suddenness of the event that will END an ongoing state of affairs. What was arriving to those on the earth was the Flood itself; it wasn't "invisibly present" beforehand.
You forgot to mention the ark being built and completed and Noah’s preaching ( a preacher of righteousness). They took no note. Jesus told the Pharisees: “ The kingdom of God os in your midst” It didn’t catch anybody by surprise, Noah was expecting a flood and the people were aware a flood was coming for 120 years. They just didn’t believe it. They were eating and drinking and rejecting the message from God. So the flood did not come unexpectedly without knowledge or warning. They were told that they were living in the time of the end. Noah didn’t know the day or the hour but he knew he was living in the time of the end. So, the flood did not come suddenly in the sense that nobody was expecting it to happen. The Parousia like the days of Noah involves a period of time, the time of the end identified by the sign of christ presence and people rejecting the good news of the kingdom until the climax when the son of man is revealed.
During the time of the end Jesus is riding a white horse so that is part of the Parousia First he came on a colt finally on a horse of war. He certainly is present riding that horse, conquering tilll a completion and other horses follow. “ Go subdoing in the MIDSTS of your enemies.” The Messiah was also invisible to the Jews. He was an elephant in the room performing miracles and doing God’s work in their midsts.
It was about Noah and the ark not about the flood. The flood was against God’s enemies, the ark and Noah for his friends. The sign of the son of man like the flood is for God’s enemies that didn’t take note.