From page 5-144 of the U.S. owner's manual ..."
Warning display screens with the “[mark indicator symbol] ” mark displayed in the upper right of the screen can be switched. If you want to switch the display, press the [menu] switch for approximately 2 seconds or more."
I'm guessing there's an alternate warning display for having the door open while in gear.
You're correct - in this case (door open while in gear), if you keep the menu button (highlighted in Yellow box) pressed for 2 secs, the display changes back to the 'normal' one with a warning sign (similar to the 2nd image below) shown on top.
In essence, it's an "ignore/dismiss warning" icon. You press the same middle button if you want to clear any other similar messages such as a "low-fuel" or "TPMS/tire pressure low" indicator, which are displayed on the gauge cluster LCD panel.
The fuel consumption calculation for the EC is set to automatic by default. This setting is noted by the number 1 in the display as you have highlighted. You can set this mode to MANUAL in the Settings screen and the number 2 will show.
Unfortunately, manual calculation mode only lasts until you turn off the ignition. It then resets to AUTO mode. The only way around this is to set the calculation mode to MANUAL every time you start the EC BEFORE engaging drive and moving off. This will take the average from the previous trip and use that as the start figure for calculation. If you leave it at AUTO  then it will start from its own preset figure each time.
I have found that the manual mode does work and you can get an accurate fuel consumption figure but having the reset the calculation to MANUAL every time you start, and BEFORE you move off, is a pain in the A...
I've given this practice the flick since and just go with the AUTO. From my reading there is a way to set the process to MANUAL permanently using the Mitsubishi diagnostic tools but convincing a dealer to do this may be nigh on impossible.
I have seen screens of a guy advertising in a Singapore EC forum who does these things for a fee out of hours, probably using the dealers gear, and the screen displayed shows the fuel consumption setting "secret" function amongst other options.
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