Can always remember looking at the first 4200 I looked at back in 2011 which was a facelift so at the time would have been 5 or 6 years old. It was rusty underneath, and if you opened the boot there was rust around all the little 5mm holes in the boot pressings. Although it had less than 20K miles it must have lived outdoors and been used in the winter with no additional rust proofing protection.
It put me off buying one at that point, Italian cars they are all the same rot boxes!
Fortunately I had a second look and bought the one I have now, that could not have been different having not been used through any winters.
Unfortunately whatever model Maserati, the majority of first owners, probably more so the GT and QP models especially being daily drivers will have been used through the winter months. No additional rustproofing protection, why would you, you probably only intend to keep a few years and move on, but damage done for future owners down the line.
Unfortunately don't kid yourself that 'rust treated' and all okay now, with a coat of wax covering it up, the only true rust treat is removing the rust permanently by blasting it away, or it will come back eventually.
My Triumph Stag was a rusty pear at 16 years old when I started restoring her. The shell however was fully bead blasted clean and repaired with new panels. 30 years later still as good, but properly protected and no salty winters.