Heard a rattle in my M.M.M.

[Bondonutz]

I'm going through my E-Revo doing some mods and changing up some stuff and removed the ESC from the chassis, I heard a rattle like a pebble or somthing in the case so I opened it up. Plunk, out falls this do-hicky. You can see where it came from via the screw driver and I have no clue what it is or whats it's for but I guess It's best that I send it back to Castle for Inspection/repair.
Funny thing is I have no clue how long this has ben like this, I last ran the truck 2 weeks ago and it was a perfect run. 50 minutes and cool temps when complete. Put the truck away and that was that. Finially received all the parts I ordered and started disassembly and found it as you see it ?

Any insight is welcomed, Thanks.