Finding a Lincoln cent sleeper is not easy. It is also not easy to identify a Lincoln cent that has doubled in price in recent years without any notice. In both cases, however, the 1952-S happens to be one Lincoln cent date that can be named to answer both questions.
Precisely what is going on with the 1952-S Lincoln cent and why are a couple very good questions. Historically the 1952-S has never been considered an especially tough Lincoln cent. In fact, with its mintage of 137,800,004, it has not even been considered an especially good Lincoln cent date from the dates produced from 1941 to the end of the wheat stalk reverse in 1958. There are a number of lower mintage dates from the period beginning in 1941 and their MS-65 prices are indeed higher.
Actually for many years it was natural to overlook the 1952-S. Both the 1954-S and 1955-S had lower mintages. In the case of the 1955-S, however, there was heavier saving. The government had announced that 1955 would be the final year of coin production at San Francisco. That fact alone was enough to cause heavy saving of the 1955-S and since it ended up with the lowest mintage for a Lincoln cent since the 1930s, that saving was only increased.
Another factor was certainly that the number of coin collectors was growing steadily during the 1950s. The vast majority of new collectors started with Lincoln cents, which meant that every year the number of new Lincoln cents saved was likely to be higher than the previous year.
It was also a case where the 1952-S was perfectly positioned to get lost in the shuffle. About the time the 1952-S was first being released, there was great excitement over the 1950-D Jefferson nickel, which had the lowest Jefferson nickel mintage in history. Suddenly people were hoarding the 1950-D and something as ordinary as the 1952-S Lincoln cent was not likely to get much attention.
In the years that followed there was really no reason for change. There were adequate supplies of virtually any Lincoln cent in the period after 1941. Moreover, at the time, collectors were not terribly concerned about top grades. They were collecting from circulation in most cases. If a collector wanted an example of any grade in Mint State there was another consideration as there were no grades like MS-65 being used with Lincoln cents. The 1952-S was either uncirculated or not uncirculated, and it really mattered little in terms of price.
Back in 1998 the 1952-S was at $2.50. In 2006 the MS-65 price of the 1952-S had moved up to $5. In 2009 it is $9. In terms of percentage, that is a very solid increase. There is really no large supply of the 1952-S. In lower grades the 1952-S might be readily available, but in MS-65 and better the 1952-S has turned out to be a sleeper but one that appears to be wide awake and moving in price.