How could Ruby miss seeing Sgt. Putnam in the front seat if he was situated between Pierce and himself? He couldn't. He wouldn't. He didn't. So, Ruby must have been there at another time, a time that Pierce was riding solo.
In the picture below, the young man, who is driving, corresponds to Pierce, and the young woman corresponds to Sgt. Putnam.
So, it's the equivalent of Ruby not being able to see her.