B737 Undifferentiated or Other Model
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2.0
Human-Machine Interface; Situational Awareness
ATC Issue All Types; Airspace Violation All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Requested ATC Assistance / Clarification
ATC Equipment / Nav Facility / Buildings; Airspace Structure; Human Factors
Aircraft X was turned on course and climbed. Approaching 5000 feet the pilot asked if his assigned altitude was 5000 feet or 15000 feet.  I said 15000 feet and he responded that traffic was descending overhead. I then noticed the target. It was a target with altitude only and no beacon code and it was Aircraft Y. I told Aircraft X about the target and said I am not talking to that aircraft. Aircraft Y keyed up and said it was him. I clicked on target and noticed it was on the wrong code; got correct code from the Flight Data Printer which is not at the position. I gave Aircraft Y the correct code and still could not track up the aircraft. I had to get the supervisor to track up the data block from the coordinator position. The session was quite busy and complex. Previously during same session I had asked for a coordinator and denied VFR flight following to numerous aircraft. Aircraft X noticing the other aircraft visually and or on TCAS and questioning the altitude was the only thing that prevented an error. I'm not sure if I accidentally dropped the Aircraft Y data tag or if the tag was never handed off from ARTCC in the first place.  I do not recall talking to Aircraft X previously; but I was very busy and cannot recall for sure. Final and departure west were both open at the time.
An aircraft was in a Controller's sector with only a 'limited' data block displayed. The Controller did not notice the target or realize the aircraft was on their frequency and flying through their airspace.