The fact that your used units stank and not the new ones make me think it might be something from the previous user's bedroom. I've smelled a Vicks Vaporub smell from one used CPAP and one had a sort of perfume smell. The smells went away pretty quickly.
I think it's really unlikely that any outgassing from your CPAP is going to be hazardous. If it's outgassing some volatile compounds out, it's depleting whatever small amount of that chemical is inside the machine. There's a LOT of air flowing through the machine during a night, around 10,000 liters. It should run out of its supply of whatever it's outgassing pretty quickly.
Presumably they didn't leave in anything seriously toxic in the plastic. Your nose is fantastically sensitive to some molecules. It seems in chemistry, most of the really nasty stuff is odorless, though. 8-(
Plastic smells, but if it's toxic, you're probably already dead from the enormous amounts of polymers in your house already. Paint, carpet, flooring, adhesives, clothing, shower curtains, plastic bags, etc.