The needle bearring cant really come out unless the wrist pin is removed from the piston/connecting rod. I have seen a needle bearring fall out and destroy a cranks con rod bearring from a guy that didnt see it fall out much like you are talking about. Look under the crank in the bottom of the cases with a flashlingt to see if its in there. A magnet would be your best bet IF it even went in there or on the floor? Its always a good idea to replace the wrist pin bearring whenever you have the top end apart. They are around $10. Good luck.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes