Take for example, bento boxes at train stations. You would think that they would put pictures of the food on the outside, or make the boxes transparent plastic. But no, they make them opaque as if trying to wrap it as a gift, and you can have no idea what's inside unless there's an example display. Very odd. I guess it goes to the theme, "you just have to know".
In my experience, the bento sellers inside train stations keep one box of each variety open and covered in plastic on the counter. Yes, it sits out all day, but no one is going to eat it, since it just stands in for the plastic models that brick-and-mortar stores use. Just point to the option you want, and they'll give you a freshly made one. (There are laws about how long a bento can stand outside unrefrigerated.) If there's a strip of paper or card with handwriting across it, that usually means "sold out." (売り切れ, oo-ree-kee-reh).