That's what I meant... I hear a controller say to a (probably IFR) aircraft something like: "VFR traffic in your 11 o'clock at 5 miles 5500 feet unverified" and I think I'm at 5500 feet, could it be me?
Was the traffic being passed to the IFR on his departure report? If so, it should have had a location and outbound track in the report. If you monitor the frequency you will hear all this information, you can then make an assessment of whether you are that traffic. If it's a random traffic report and you hear it:
'Centre, ABC, VFR, 10 miles south of xxx tracking east, A055'
'ABC, Centre, sqwark ident'
Should I speak up?
Why not? Remember, it wasn't ASA or ATC that encouraged VFR's to shut up and say nothing. I'll get banned (again) if I say who's bright idea that was.
I have no idea where the aircraft referred to is. Usually it turns out they're 50, 100 miles away or more.
'ABC, Centre, thanks, no observed traffic, area QNH 1013, have a good day'
I have no problem ignoring it now, but the first few times I was unsure because of the rule that VFR traffic are supposed to listen and announce if they may be in conflict.
That's entirely up to you and your judgement. Your risk assessment in other words I suppose.