Someone posted this also few days back and found its location and could be where yours has come from.
Remove hotend cover, remove nozzle, you should see 3 screws holding the heating element in place, unscrew them and remove the heating element and turn it around so the back of the heating element is facing you, there are 4 screws there, thats where another users screw had come lose and fell from. I hope this helps.
Edit: Heres the link to who found where it came from, your screw looks identical.