1 Thessalonians 5:2
for you know perfectly well that the day of the Lord [i.e., when He returns, See II Thess. 2:1-2] will arrive like a thief coming in the night.
2 Peter 3:10
But the day of the Lord will come as a thief [i.e., unexpectedly]. At that time the heavens will pass away with a loud noise and the elements [Note: This refers either to the heavenly bodies or to the material components of the universe], will be dissolved with intense heat. And the earth and its works [i.e., its activities] will be burned up. [Note: Some translations render this phrase as it is in some ancient manuscripts, "will be disclosed," or "will be laid bare"].
Revelation 3:3
So, remember how you have received and heard the message [i.e., they had eagerly accepted the Gospel]; [now] obey it and repent. For if you do not become [spiritually] alert, I will come as a thief [i.e. unexpectedly] and you will not know the [exact] time that I will come upon you [in judgment. See 2:16, 25].
Revelation 16:15
[Note: At this point the Lord's voice breaks into John's vision]. ("Look, I am coming like a thief. That person is happy who stays alert and is dressed [properly], so he does not appear naked and have to be shamefully exposed").
Matthew 24:42-44
Therefore be alert, for you do not know on what day the Lord will come.
Matthew 25:13
Therefore, you should be watchful, for you do not know the day or hour [i.e., when the Son of man will return].
Mark 13:34-35
It is like a man who left his house on a trip to another country. [Before leaving] he put his servants in charge of his affairs, giving each one a [particular] job to do, then he ordered the gatekeeper to stay alert [i.e., while he was gone].
Luke 12:39-40
But you should realize this, if the owner of the house had known exactly when the burglar was coming, he would not have allowed his house to be broken into.
1 Corinthians 1:8
He will also make you stand firm [in the faith] to the end [of your lives], so that you will be [considered] without just blame on the day when our Lord Jesus Christ [returns].