Dont' you guys have telepathy courses dunnunda ?
Because that might be useful to provide any service to someone you don't have in contact...
In fact, I can think of one case of providing a service to a VFR not in contact, and that would be to have military activity stopped if you see that someone (with a transponder) is entering a restricted or danger area. That's the kind of service the guy in the plane might appreciate especially if they 're firing some missiles or shells...
If we talk about some Vfr in E or G airspace that contacts you, you owe him flight information service (not control) and alert service, and that would be in theory weather, active zones, volcanic activity, ... traffic information being still recommended even on unreported traffic that you've detected ! Trouble is that the last point would probably create a false sense of security ( and actually of control) from the vfr point of view. One would probably think that the controller has everybody in contact, which is definitely untrue, but if you don't signal traffic that you have seen, your responsability might be engaged...