Originally Posted by Rob Helmerichs
Although if you think about it, it would have been insane for him to hide it in the family car on his way home. And Dexter is not that kind of insane!
That wasn't necessarily the intended or expected assumption. Most of us probably didn't really think about it that much but if we had... some of the natural assumptions would have been:
1. He's got it in the car, but he's going to go by the pharmacy and take the drugs to the house to drop them off and be on his way. His lack of awareness of family matters would have made him believe that he'd be able to just drop off the drugs and head off to the boat with some typical excuse.
2. Maybe he thought he'd store the body parts somewhere else till he could dispose of them the next day or night, as there was little reason to think the training center would be the acceptable place to leave them.
3. He was so damn tired that maybe he thought he'd just keep them in the van till he could get rid of them at first light, instead of going to work. (this probably being the most expected theory and corresponding to your point.)
In any case, any of these would have resulted in the body parts being in the van and subject to discovery, especially when Dexter himself was having thoughts/concerns of this nature.
People who are suffering from severe sleep deprivation can do things that appear to be insane but are really just results of temporarily delusions. Maybe even temporary insanity if you want to put a common label on it.