The 20x4 LCD Displays allow 'custom characters' to be defined. The corners of the box are customer characters. Marlin also uses custom characters with the UBL Mesh Edit from the LCD Panel. It uses them to construct a grid of the mesh points and to show you which mesh point you are editing. lcd.write('\x00'); through lcd.write('\x03'); are the custom characters needed to make the corners of the box.
Just removing the lines within the logo_lines() function will get rid of the Marlin logo. But why would you want to hide the fact you are running Marlin firmware?
Just removing the lines within the logo_lines() function will get rid of the Marlin logo. But why would you want to hide the fact you are running Marlin firmware?