The police didn't check Yang Yi's trunk after all.
The reason why Yang Yi dared to throw a living person in the trunk amidst the heavy police blockade was because he was certain that the police would not inspect his trunk.
This was a sudden crisis. The identities of Brian and the other two men were clear: three white men armed and extremely dangerous. The police had to establish a blockade and conduct searches as quickly as possible, and they also needed to evacuate civilians. They didn't have the time to check the trunk of every car.
Of course, Yang Yi couldn't be sure that the police wouldn't inspect his trunk, but how could one be completely certain of anything? As long as he felt there was a possibility of success, and the odds weren't too low, that was good enough.
This time, Yang Yi's judgment was correct. The police really just glanced and let him leave, because three people couldn't hide in that trunk.