The pre-Trib Rapture is the event in which Christ returns to the earth but stays in the air and on a cloud. He will return to remove all His saints/overcomers from the earth. The Second Coming (the Second Advent) is the event in which Christ returns to the earth (with the clouds – with His Host of raptured saints/the armies of Heaven). He will return to bring the Tribulation Period to an end by defeating the Antichrist and his evil world empire in the Battle of Armageddon.
The Bible neither suggests/states that the Rapture will be unexpected for EVERYONE nor does the Bible suggest/state that the Day of the Lord will be unexpected for EVERYONE. On the contrary, Christ Jesus clearly implies that the Rapture ONLY will come as a thief in the night to the unprepared and unwatchful believers. Likewise, the apostle Paul clearly states that the Day of the Lord ONLY will come as a thief in the night to those individuals who are in “darkness”—to those very surprised ungodly and immoral unbelievers!