Sct TRACON controller reported an unidentified aircraft not in communication with ATC and no mode C ...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1355191
Time
Date	201605
Local Time Of Day	1201-1800
Place
Locale Reference	SCT.TRACON
State Reference	CA
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Make Model Name	Small Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Other Instrument Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Aircraft High Wing 1 Eng Fixed Gear
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	None
Flight Plan	VFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 4.5
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

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.