Would you tell the police about suspicious activity from a police officer?

I was outside with a couple friends when a cop drove by. When he got to the stop sign at the end of the block he opened his door and threw onto the stree a black garbage bag that was tied up. After he drove away my friend grabbed the bag and just as was grabbing it the cop drove by again then sped away. We took it inside to see its contents and it was a pair of men's underwear. No stains on it or anything. In the 20 or so minutes since the cop drove by twice more. What would you think/do in this situation