Sure, everything you said is correct. But the alleged drug dealer will probably face an IRS audit and get charged with tax evasion, based on the fact the he has an unusually expensive car. The car's existence will provide investigators with a path to follow in tracking down the suspect's finances, laundering activities, income sources, etc. It's not the entirety of the case (like you said, not enough to charge you with a crime), but it is a clue that an investigator can launch from.

Just because something isn't proof, doesn't mean it isn't evidence. And if I am doing something my government disproves of, I'm going to try and be mindful of all evidence that can expose that. (Note that I'm NOT making any value judgements here) If an inner-hidden volume isn't as hidden as I thought it was, then that's a security risk to me.