I recently upgraded from 1.5.5f with Classic Responsive clone to 1.5.7c with ZCA Bootstrap clone. During the transition, I had the cart in maintenance mode. In the 1.5.7c cart, I was expecting to see a header alert announcing the maintenance mode but it wasn't there. I looked at the page source and found it there. Then it occurred to me that it was probably hidden behind the navigation bar.

This was confirmed when I moved the header alert code down the file to between the navigation display and the branding display, and the header alert showed up on screen. Other header alerts are now visible, such as when products are successfully added to the cart.