Tiny little spring came out when disassembling.

yup... it is the spring for the belly panel. for the little slide switch underneath to release the plate to put batteries in.. it sits loose with little plastic slide lever in its position if controller is upside down , then as the bottom half of the chassis is lowered down onto the top half, if done carefully, will set snuggly into position and neatly trap the spring and slide lever piece in position between the two chassis halves (which must be secured together by the appropriate screws before flipping the controller upside-right)