Actually, in the case of Camus it's a bit more than a rumor. An elderly pastor by the name of Howard Mumma wrote a book in 2000, forty years after the automobile crash that took Camus' life, in which he claims to recount a series of meetings with the famous writer.
Camus was allegedly open to the answers Christianity offers to questions about the meaning of human existence and was so intrigued by his discussions with Mumma that he actually requested to be baptized.
Camus died before the baptism occurred, but, according to Mumma, he died a theist and possibly a Christian.
The book is titled Albert Camus and the Minister. Only the first 90 pages or so deal with the author's conversations with Camus, and I have no idea whether Mumma was accurately recounting those conversations.
It does seem, however, that they had some sort of relationship with each other that Camus was interested in developing.
As for Sartre, who died in 1980, there seems to be reason to think that he, too, may have repudiated the atheism he embraced throughout his adult life.
From the link:
The one who revealed Sartre’s astonishing change was his friend and ex-Maoist, Pierre Victor (A.k.a. Benny Levy), who spent much of his time with the dying Sartre and interviewed him on several of his views.Others have vigorously disputed Victor's account, and the truth will probably never be ascertained. It is the case, however, that his long time paramour, Simone de Beauvoir, was angry with Sartre for some sort of betrayal in his last days, referring to a "descent into superstition" and the "senile act of a turncoat."
According to Victor, Sartre had a drastic change of mind about the existence of god and started gravitating toward Messianic Judaism.
This is Sartre’s before-death profession, according to Pierre Victor: “I do not feel that I am the product of chance, a speck of dust in the universe, but someone who was expected, prepared, prefigured. In short, a being whom only a Creator could put here; and this idea of a creating hand refers to god.”
Whether she was speaking of a death bed conversion to theism or not is unclear.
There are lots of urban legends of death bed conversions of atheists, and what they all have in common, unfortunately, is that there's no way to confirm them. It's best, therefore, not to give them more weight than the evidence allows.
Nevertheless, the evidence in both cases is suggestive.
A line from Camus is intriguing. One source claims that in his novel titled The Happy Death, Camus wrote that "Death has the power and audacity to change one's beliefs."
If the quote is accurate it may also be prescient.