"Unless the blasting caps were inserted into the plastique with a means to generate the pulse of energy, there's nothing to "disarm.""
It still makes one wonder what it was doing in the car.
Probably storage. Heck I wouldn't want to keep the stuff in my bedroom closet. I know it's not terribly volatile, but it'd still make me nervous and I'm trained to use C4. Imagine how antsy the bozo was that had the stuff.