Primary Radar will usually pick up unknown traffic.
I say usually, because it is subject to a lot of things - for example, the CG primary radar wasn't working for around 12 months recently, primary radar often picks up all kinds of things that aren't there, and some aircraft have too small a profile or reflective surface to be detected (esp fibreglass / composite).
Also, primary radar only exists around Capital City Primary CTR's.
But it does save a lot of VCA's becoming a whole lot worse.
From my observations around 40% of traffic operating OCTA around the BN area does not use a transponder (whether not fitted or just not switched on). (Having it on even when OCTA has saved a lot of close calls with other aircraft awaiting clearance or leaving CTA on descent over the years)