Not true. Per 1 Thess 5:2-6 — especially v.4 — believers will not be caught unaware.
2 For you yourselves know full well that the day of the Lord will come just like a thief in the night.
3 While they are saying, "Peace and safety!" then destruction will come upon them suddenly like labor pains upon a woman with child, and they will not escape.
4 But you, brethren, are not in darkness, that the day would overtake you like a thief;
5 for you are all sons of light and sons of day We are not of night nor of darkness;
6 so then let us not sleep as others do, but let us be alert and sober.
Don't feel bad about it, though, I grew up in church and I wish I had a dollar for every time someone repeated that line about not being able to know. I don't know who started it, but the Bible doesn't support it.
Jesus himself flat out commanded "keep watch, and pray." What sort of fools errand would he be sending us on if we weren't going to have any way to know?? And what of the parable of the fig tree in Matthew 24? What sense would it make for Jesus to tell a parable about how to tell the time of the end if there'll be no way to tell the time of the end?
Nah, if you are counted among the "brethren" you'll know when to look up.
We recently talked about this in our adult Sunday school class. We may not know the day or hour, but we will defintely know the season. We’re not going to be caught in the dark, if we look to the only Light.
Watch, therefore...
Oh, I’m watching, Lord! Watching and praying.