In nearly 30 years of being in the towing business, I've probably witnessed a thousand police searches of vehicles.
It's actually rare if they don't search a vehicle that's being ordered towed.
A very common scenario is when a car is stopped and found to be unregistered or the driver unlicensed/suspended.
The cops call for a tow and search the vehicle under the guise of taking "inventory".
And if they just happen to find something illegal, they confiscate it and add additional criminal charges against the driver.
Happens all the time, even for many vehicles that are towed for parking violations, without an operator present.