What I do find is that about 99% (no kidding) of the "altitude unknown" contacts reported to me by the radar controller and which I manage to get visual with, turn out to be flying very low.
Obviously, my first reaction to a report is to look around my own level, plus or minus say 30 degrees, and then only if somewhat bored I start scanning the ground below... or sky above.
How low is hard to judge and "low" traffic appears a lot lower than it is but my guess is below 1000ft AGL and that does tend to correspond to the "ultralight" community. But they are powered and can power a transponder.
I have never seen an "altitude unknown" contact at my level or higher. Make of that what you want. My take on it is that most of the "civil liberties" crowd which insists on avoiding transponders flies at low levels. I suppose this is better than having them fly at 2000ft plus or in IMC; one has to be grateful for small mercies
It also means that one can dramatically reduce one's chances of a mid-air, and the stats support this.