I never thought about a customer thinking that today's hours that are displayed, are the only times you are open.
I'll ask the Google Local team if they can add a small link saying "click for complete hours" or something.
I'm sure they don't show all the hours because of display space. Google's UI is so minimalist and subtle, that for the average non-techy user it's easy to miss things like the fact you need to click hours, to see all the hours. (Although I always thought that was fairly obvious and intuitive.)
"Not sure of the best course of action - removing the opening hours from the listing probably risks affecting our Places ranking"
Want to warn that you don't want to remove hours from the listing. Don't think it would affect ranking, but could get the listing removed or suspended. Because businesses that don't have hours listed could imply they are either online only or by appointment only or closed to the public. All of which are listing types that are not allowed on Google maps.