The 4chan Party Van is a slang term for FBI agents. By either referring to the van or posting an image macro about it, a user is pointing out to another that the FBI may have noticed the illegal or questionable content of their post and are on their way to arrest them. One of the main reasons for it is that "Party Van", when read backwards, says "Navy Trap".
According to Lurkmore, the Party Van was first seen in an MS Paint comic posted to 4chan in early 2006. However, no archived mentions exist earlier than December 4th, 2006. The idea evolved into image macros, where people would photoshop images of vans, trucks, and buses to be the Party Van. The images would be used in a conversations with inappropriate content, similar to how Pedobear is used in CP threads.
The first YTMND page for the Party Van was created on March 15th, 2006. The word "partyvan" was added to Urban Dictionary on September 26th, 2006, stating that "the term is often used as a warning against illegal activities on the internet."
Party Van transitioned from a simple warning to a plausible reality with the news of the arrest of Jake Brahm, a 20 year old Wisconsin man. He was apprehended by the FBI for posting faux threats on 4chan about detonating bombs at seven NFL stadiums on October 22nd, 2006. The threat, which Brahm referred to as "America's Hiroshima," was reposted approximately 40 times across /b/ and other websites.
The FBI took the threats seriously, tracking down Brahm's IP address and taking him into custody. Brahm's lawyer "urged leniency for an "unsophisticated kid" who was immersed in the 4chan.org culture." In June 2008, he was sentenced to six months in jail followed by six months of house arrest. It was physical proof that the FBI was in fact looking at 4chan and could take real-life action on users, despite their anonymous posting.
The /i/nsurgency W/i/ki, founded on June 7th, 2007, was put together as a central source for information on raids. They adopted the term Party Van as being their own form of authority coming over those who were violating their rules of the internet.
 Freedom Eden – Jake Brahm: How To Screw Up Your Life Without Really Trying
 Urban Dictionary –