The button is the menu button and its pic looks like a few sheets of paper stacked loosely. From the factory, it is the left most button on the top row of buttons on a RHD vehicle. Not actually visible by the driver, but is there none the less. I had mine moved to be visible at the first service, to the most right button hole on the top row. A few minutes by the service tech.
There is a flaw in the design as if you have a trip screen viewed and it is overlayed by another, and you are holding in the button to bring it back, it would be possible to clear the trip data by mistake. You actually have to concentrate too long on the dash and not on the road. There should be a button on the steering wheel, quite separate to the menu button for this purpose. My PHEV aspire has several blank button spots on the steering wheel where this button could live. Come on Mitsubishi, get the design team to work on making the drivers interface better, including the ridiculously small fonts used to display range data on the MMCS.