What is this stamped metal piece that fell out of a new hydraulic shifter? I’m finishing setting up a new bike, and right as I finished bleeding and shifting around the left shifter (r7025), this little stamped steel bit fell out. 
It’s about 11mm in diameter and 0.5mm thick. I have no idea where it came from beyond “somewhere in the left shifter’s mechanism.” I couldn’t see anything obvious on the right shifter that looked similar and was hoping someone could ID this thing and might know where it fits back into the assembly so I don’t have to mess with it more than necessary.
For reference: it fell out as I finished positioning the brake lever after bleeding. It doesn’t seem especially related to anything hydraulic.
Providing the 7900 is the same, I’m sure it is but someone more qualified can chip in if it’s different.

How did it fall out ? Shimano have a proper tool for removing them.
Info from here Fawkes