My first clearance was granted in 1986, and of course I had to list a number of people as references. Most of those people reported back to me that they had indeed been visited by Federal agents to confirm the information in my clearance application, including one who was a bit freaked out because he got his visit while he was tending his illegal marijuana farm. He said the Feds pretended not to notice and apparently did not report him.
Subsequently during my career I had to go through the process several more times at ten year intervals for what is called Periodic Reinvestigation. Because of the time intervals and my changing states of residence, the list of people I gave as references changed. None of the people I listed as references for the reinvestigations ever reported getting a visit. I found that odd.
I’ve never held a security clearance, but my father-in-law did for way over 40 years. My wife and I lived in another state soon after we married, I was told by some of the neighbors that they were questioned about us, they wanted to know why.
This was in the 1970-80’s. I can assure you that it was taken seriously then.
My original clearance started 1979-1980 the investigator did come to the university where I was a grad student. He was walking down through the grad student cubicles showing people a photo & asking questions. (Even then I was like one of a handful of Americans!) My photo must not have looked much like me (or me not like it!) because he didn’t recognize me and started asking me questions. I laughed & told him he’d hear nothing good from me about me, showed my student ID. We had a chuckle about my picture then I left him to do his business.
Mine was 1967, and I just stumbled on my old application—and it runs for pages and pages!