Just because your vehicle has been towed, that doesn't necessarily mean it's been impounded.
Denver, unlike many municipalities, doesn't automatically impound towed vehicles. Vehicles that are towed are often relocated to a nearby location and parked legally there. There is a $100 tow fee incurred for moving a vehicle.
If your vehicle's been towed, contact the Denver Police Non-Emergency line to locate it: (720) 913-2000 (when you get the recorded message, hit 0 for the operator, they will look up your vehicle's location)