My grandma, born 1933, was telling stories from her childhood when she mentioned that her father was visited by government officials during the war. She also remembers him being questioned in more detail, and possibly interrogated at other locations. His name last name was Hess, he lived in the Midwest at the time, but was not politically active or in contact with anyone in Germany. Was the US Government simply questioning all Germans? What else could have attracted interest from the government?
The FBI did investigate many people who were of German origin during World War II, fearing they might be "fifth columnists" working for the German state. They even interned several thousands of them in camps. As for why they might have looked at your father, they did investigate a huge number of cases, but he might also have gotten on their radar either through past acquaintances (he didn't have to be politically active, he might just have had friends who were) or confusion of identity (in my experience in reading FBI files, they devoted a huge amount of resources in the 1940s and 1950s to following up on cases of people having the same name as other people but were entirely unrelated*, and Hess is a very common German surname, so that's entirely possible).
It is possible, though not extremely likely, that the FBI still has files on your great-grandfather. You can submit a Freedom of Information Act request to them — send them the name of your grandfather, his birth and death dates if you have them, and proof of death (though his age is probably proof enough, but if you have an obituary or a record in the Social Security Death Index, it will make things smoother). It takes a few months for them to get back to you (which is light-speed by FOIA standards). The most probable answers they will give are, "we have nothing under that name," "we might have had something under that name but our records say we destroyed it" (common for old files of no consequence), or "we gave these records to the National Archives and Records Administration so you'd better take it up with them" (which would mean you'd be submitting another FOIA to NARA which will take years and years to process). If you're very very lucky (which is probably only likely in the case that they actually cared a lot about your great-grandfather for some reason), they might actually have a file.
*My favorite example of this comes from the file of Edward Teller, a vigorously anti-Communist Hungarian physicist who worked on the American nuclear weapons program. Apparently there was another person named "Edward Teller" who had taught a course on Marxism in New York City at some point. Despite the fact that they had nothing to make them think these were the same people except for the coincidence of naming, they spent quite a lot of manpower on the question of whether "Edward Teller, anti-Communist physicist" was the same person as "Edward Teller, pro-Communist teacher." Given how outwardly anti-Communist Teller was, and how many security checks he had to go through to get his position as a nuclear weapons designer, it is rather humorous that they would be worried that he might have moonlighted as a Communist teacher at some point, but they weren't going to leave any stones unturned.