If you find drugs, I'd recommend leaving them in the truck and calling the police. You have records that you purchased the drugs from them, via a vehicle sale. Also, I don't think any cop would assume it's yours. No druggie calls the cops for them to come collect their drugs LOL.
Speaking of drugs, I was talking to a friend who lives in Colorado and is a cop. Apparently due to weed being legal there, companies are coming out with a wax THC extract that is like 80+% THC. They bake them into edible stuff, like cookies and gummy bears, and if you aren't careful you'll trip harder than a hit of LSD. A guy is right now facing homicide charges for shooting his wife. He was a regular family guy and decided to try the edibles and ate too many. So apparently weed does kill, when taken to the extreme.