Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 4.5
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Separated Traffic; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Airspace Structure; Procedure
Overcast day with low ceilings. Working multiple approaches to multiple aircraft to multiple airports. Aircraft X was cleared for an approach descending to 2800 feet until established. I noticed Non Mode C traffic about 5 to 6 miles ahead and called traffic with altitude unknown. Aircraft X responded 'IMC'. I called traffic again a couple miles later and informed Aircraft X targets appear likely to merge. Aircraft X informed me they were just coming out of the clouds and searching. Normally they would already be shipped to the tower but I kept him on my frequency in case I had to abort the approach. The tower called when the targets were a few miles apart and told me the last know altitude of the target was 1700 feet. I informed Aircraft X of the last know altitude and he informed me he had the traffic in sight. I then switched him to the tower. I was informed a few minutes later the target was Aircraft Y and he had a waiver to fly without Mode C. Aircraft X called my supervisor to ask how did we know the Non Mode C aircraft's altitude; what he was doing there; and how come he didn't get an RA if we could see his altitude?I don't think anyone should be flying around the Mode C veil without a transponder; especially under a Class B airspace. I'm not sure who would issue a pilot a waiver for that. It is very dangerous and a completely unnecessary risk in such a technical airspace. I would also make the Class Bravo more restrictive to reduce the amount of IFR to VFR issues.
SCT TRACON Controller reported an unidentified aircraft not in communication with ATC and no Mode C crossed the final approach course of an inbound aircraft on approach.