Fictional town in Germany. No roads lead to Bielefeld, no railways either. It does appear on maps, but no-one has ever been there or knows someone from Bielefeld.
It might be a code name for some secret militairy project.
Everybody who claims to be from Bielefeld or to have ever been there is part of the Bielefeld-conspiracy (German: Bielefeld-Verschwörung)
X: Have you ever been in Bielefeld?
Y: Sure, I was there only about two weeks ago.
X (to Z): Never trust Y again, he part of the Bielefeld-conspiracy