I have no idea. Never touched a Fantom (or many other vacs, for that matter). I would just look up in that hole where it fell out of and see if it makes sense to be there. If you can't determine anything, just put the plastic bit in a drawer somewhere until such a time as you can figure where it goes, if it even goes at all.
If it is what I think it is, its absence will be harmless unless you're sticking screwdrivers or fingers into places they ought not be with the machine plugged in.
