The United States Congress has only formally declared war 11 times, doing so on eight dates, for a total of five different conflicts. Six declarations on three dates were against various foes of WWII, two declarations on two dates were for WWI, and there was one appiece for the Spanish-American War, the Mexican-American War, and the War of 1812. It's sometimes difficult to prove a negative (such as that Congress never did something) but in this case it's such a rare event we can simply look at the record and say no - it was a pretty well known fact each time that it was time to fight.

WWII we entered after Pearl Harbor. It was big news around the country, people were calling for blood, and the the declaration against Japan was pretty much a given. Germany and Italy declared against us a couple days later, and we responded in kind with formal declarations against them. Other Axis countries garnered the three remaining declarations some months later.

WWI was entered after a relatively long period of being neutral, which was eaten away and finally mostly quashed by the Zimmerman Telegram, in which Germany offered to fund and assist Mexico in reconquering states in the American southwest, in the event the US attacked Germany due to Germany's commencement of unrestricted submarine warfare and sinking of American ships. It was intercepted by the British, shared with the US, and published in the media. Once it was confirmed to be genuine, and some US ships were sunk, it took less than a month for war to be declared with widespread public knowledge and support. It was initially against Germany, with a second formal declaration against Austria-Hungary half a year later.

The Spanish-American War was not only a widely popular cause (related to the sinking, probably accidental, of the USS Maine), the war was arguably the result of tabloid sensationalism and public pressure more than anything else. Suffice to say, it was definitely not concealed from the citizenry.

The Mexican-American War came about as a result of the US annexing Texas, which had broken away from Mexico, but was not recognized by Mexico as independent. Mexico saw it as a violation of their sovereignty when we sent troops in to Texas, and we decided to pick a fight. This was a major point of public debate itself, with accusations made that it was an effort by the slaveholding states to expand slavery, was imperialistic, etc. Certainly the addition of Texas and resultant conflict were not hidden from anybody.

The War of 1812 has very complicated origins, and no one trigger can be easily identified, but issues of impressment, sovereign authority and honor, free trade, and British attempts to create rival Native American nations to wage a sort of proxy war all lead to great public demand for some sort of reckoning, which Congress obliged. Nothing kept secret here. This was the first formal declaration, and so is the last on my list.

Info on every Congressional declaration of war is gathered from Congress itself.