The idea of Hitler having escaped Germany is something that has existed in the imagination of a lot of people after 1945. Throughout the Cold War, in particular with the ever increasing reports of Nazis feeling elsewhere and apparently doing 'shadow-y' things, there was a constant stream of sensationalist articles published throughout Europe (and the world) telling about how Hitler escaped, that Nazis actually controlled the Egyptian government or other claims involving Nazis doing things that could effect the world as we know it. In the realm of popular fiction and conspiracy theories, this manifested itself in concepts such as Nazis and the occult, Nazi UFOs and secret weapons.
One of the main reasons for this belief is the fact that we do not have a grave or a picture of a physical body. The fact that Soviet archives were closed and the fate of whatever was left of Hitler was sealed shut for many, many decades only added to the mystique.
Whenever people ask if Hitler managed to escape to Argentina and what not, they always refer to the FBI files. Few people bother to actually read them. The FBI file on Adolf Hitler after 1945 is not the FBI's investigation of Hitler's possible escape - it is instead filled with reports from ordinary people that the FBI has received and compiled.
Here are some of the sightings of Hitler included in that file:
.... And these are all just from part 1 out of 4!