I had a 2015 Explorer Limited and never smelled it. Put 43000 + on it. My son has the same vehicle and has smelled it. I now have a 2017 Explorer XLT and in 2000 miles have not noticed any odor yet. (Knock on wood).
Do you and your son drive the Explorers in the exact same manner? Meaning does either one of you drive them occasionally at wide open throttle and maybe the other driver does not?
I think this problem is semi complex, especially regarding untreated carbon monoxide. Unquestionably the police interceptors are driven hard and taken to their limits, so if there is faulty exhaust manifold with holes or gasket leaks they would be first to figure this out vs civilian Explorers who don't drive their Explorers to the limit. Perhaps this untreated carbon monoxide enters the police interceptor cabins through leaks caused by modifications or maybe through leaks in the cabin that all Explorers have, not sure. The public really needs to understand what is going on conclusively soon because if there are faulty manifolds/gaskets or exhaust leaks in the engine bay and untreated carbon monoxide can infiltrate the cabin from the engine bay, civilian owners could discover this same issue if and/or when they have to drive their Explorers at wide open throttle or when they drive their vehicles to the limit. No one wants to own a vehicle that cannot be driven safely at wide open throttle even for emergency purposes only.
As far as treated exhaust out of the tail pipe with much lower concentrations of carbon monoxide, I'm in hopes that's a vacuum issue that will eventually be taken care of in the near term with sealant modifications and/or ventilation fixes.
The last possibility is that both treated and untreated exhaust emissions are entering the cabin from the rear and elsewhere throughout the body where natural cabin leaks exist.