Well, it wouldn't hurt to call a tip in to the police. But keep in mind a few things: first, your husband or you did not actually see anything happen. Second, situations happen like this often, and when it comes to an issue like drugs, police are often too backed up to really check it out unless they hear about it multiple times. Third, there's really nothing they can do without reason to believe something actually happened.
If I were you, I'd make a note of it, and write down every time you see something. Write down what you see, what time, and see if you can get tags or car descriptions. You can call the police and leave a tip, too, just in case they've gotten calls from other people and are investigating. Good luck!