Actually, the same dilemma exists all around us. For example, you know for sure that food prices will jump with inflation, and you suggest to loved ones to stockpile a few items, but they don't listen. So, should I jump in their home after midnight like Santa and stash a few cans under their beds or sofas ??

People need to be educated first. As long as they do not accept the idea, there seems no point in hiding something in their pockets.

And there is another point
Why have you chosen a method of SECRETLY adding things in their cars instead of just requesting them to accept the gift and please put in the trunk, just in case . I think this would be a more logical way.