Reading my Osprey book on WW2 German police forces, it mentioned how the SS infiltrated pretty much every security and/or public employee service that existed, even the postal service. It mentioned that factory security guards had to wear uniforms and report to the local Orpo.

So I wondered if Germany had private rent-a-cops, even if they did have to report to the government. If so, did they wear police-style uniforms the way American security services, both private and public, generally do? I suppose it's a matter of how much private enterprise was allowed to exist in Nazi Germany.

What I learned about the basic difference between Naziism and Communism was that Communists took direct ownership of the means of production, while their rightist counterparts left many of these entities in private hands, but exerted tight control over them, dictating wages, working hours and conditions, acceptable output and margin of profit. Some have called this kind of control "dirigisme."
This being the case, it really wouldn't surprise me to hear that private security contractors were employed by the Nazi regime: what difference would it make, if the forces provided were state or private employees, as long as they were effective and dependable? On the other hand, the Nazis loved imposing officiality on everything - this might date back to the old Prussian junkers' contempt for money and the bourgeoisie. Service wasn't about a contract or a paycheck - it was something sacred, bound by oaths and covenants. The Nazis weren't just about getting results, and they did a lot of things we still can't make sense out of. But your question is still very interesting.
Maybe authorities in some of the occupied countries in Western Europe, where the culture was different, would have been willing to deploy armed private detectives to keep watch over factories or infrastructure, if too many of their soldiers were still in custody as POWs, even if the Germans themselves wouldn't use them.