Yes, over the years I've met some people who not only claimed to be atheists, but also sincerely believed that God did not exist.
But notice that I said they "believed" God didn't exist -- because if they were honest, they had to admit to me that they couldn't prove it. The best they could do was say that they had never found Him. But what if they'd been looking in the wrong place, or in the wrong way? In other words, atheism is based not on facts, but on faith -- a faith that God doesn't exist.
Like you, I've found that some people who claimed they didn't believe in God weren't really sincere; they just thought it made them sound smarter than people who believe in God. But more often, I've found, they didn't believe in God simply because they didn't want to -- because they wanted to be free to run their own lives.
The Bible warns, "The fool says in his heart, 'There is no God'" (Psalm 14:1). Why does the Bible say an atheist is a fool? One reason is because someday every one of us will die and meet God, and they will be left without hope. But the Bible also says atheists are foolish because God has done everything possible to reveal Himself to us. He did this most of all by becoming a man: Jesus Christ. Do you want to know not only that God exists, but also shat He loves you? Look at Christ.