I've attached two photographs taken during my ride across northern Europe in late 2003. I camped alongside an MUP in the sand-dunes near Zandvoort that was ultra-busy during the day. The first shot is from the path side, the other is my tent all set up for the night. I heard people walking, running and riding along the path, even with dogs. But none knew I was there. The jogger next morning who obviously liked going "off-road" and ran past my tent door was the only one who did.
The third shot is of my campsite alongside a brook, with a school on the otherside, in the Champagne region of France. It was a narrow fit. Everything was great at night, until a man out front next morning said: "Padonez moi, monseiur". He was an angler trying to get along the path for a local fishing competition!
The fourth is of my tent set up in a wooded gully next to a paddock in southern Holland. It was eerie at first, but no-one found me, or at least bothered me.
I found on that trip that remnants of unfenced forest remain in various parts of France, Belgium and Holland. I would ride until dusk, pick a copse, move in and set up. I was far enough off the road for car lights to never see me, and therefore I could stay out of the occupants' site. However, outside Liege, I hadn't counted on the MTB riders and their dog riding through that particular wood. Still... no problems.
On my first night in Belgium, I had just got off a ferry from Dover and wsa trying to connect up with the North Sea Cycle Route. I was in an industrial area, and found a disused train track behind a dune -- the hillock was the barrier between me and the roadway. Once I had figured out how to peg down the tent between the rails and sleepers, I was fine. There was a fair bit of truck traffic on the road over the dune during the night, and I woke up suddenly in a cold sweat after dreaming the railway line was not in fact closed, and a train was bearing down on my tent!!! After overcoming that fear, I slept the rest of the night like a baby.
I think that if you are discovered, and if you look like a serious camper with a decent tent (as opposed to a ramshackle collection tatty tarps and the like and rubbish scattered about -- and no, I am not swiping the hammockers), European people will accept you for what you are and leave you be. I am not so sure about big city Americans or Canadians, for whom everything needs to be controlled. In rural areas, it might be a bit different. In Australia, you'd likely get: "You right mate?! Here, have a frothy!!"
