Matthew 24:50
that slave's master will return on a day that he least expects, and at an unknown hour.
Matthew 24:42-44
Therefore be alert, for you do not know on what day the Lord will come.
1 Thessalonians 5:2-3
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.
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].