S46 APCH CTLR DID NOT NOTICE AN IFR C182 DATA BLOCK DROP OVER THE RADAR ANTENNA; RESULTING IN A CONFLICT WITH A DASH 8 ON DEP W OF THE ARPT.

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I WAS WORKING SEATTLE APCH; ARR W; WITH MODERATE TFC. I TOOK A HDOF ON THE C182; 5 MI E OF SEA VOR. I VECTORED THE ACFT FOR DOWNWIND TFC. THE C182 WAS AN OVERFLT; E TO NW; AND I TURNED HIM TO 250 DEGS BEHIND TFC. THIS ACTION PLACED THE ACFT IN SUCH PROX TO THE RADAR ANTENNA THAT HIS TRACK WENT INTO COAST MODE AND HIS DATA BLOCK DROPPED. DUE TO VOLUME AND WHAT I BELIEVED TO BE HIGHER PRIORITY DUTIES; I FAILED TO ADVISE THE DEP SECTORS OF THE ABSENT DATA BLOCK. THE DATA BLOCK REMAINED IN COAST FOR 1 MIN. IN THE MEANTIME; THE DEP W CTLR IDENTED A DEP OFF OF SEATAC ARPT; A DASH 8. THE DEP CTLR SUBSEQUENTLY ELECTED TO TURN HIS SWBOUND DEP N ON COURSE TO PAE; THEN YVR. THE LCL ORDER; 546 7110.65A; GIVES THE DEP CTLR AUTH TO CLB IN ARR AIRSPACE WITH RESPECT TO TAGGED TARGETS. IT IS INCUMBENT ON THE ARR CTLRS TO ADVISE THE DEP CTLRS OF ANY UNTAGGED TARGETS IN THEIR AIRSPACE. WHEN THE C182 REACQUIRED W OF SEA; THE FINAL CTLR CALLED MY ATTN TO THE FACT THAT THERE WAS A SIT DEVELOPING IN MY AIRSPACE WHERE THE DEPARTING DASH 8 WAS APCHING PERILOUS PROX TO THE C182. I IMMEDIATELY CALLED THE DEP CTLR AND ADVISED HIM OF THE TFC SIT. I THEN ISSUED TFC TO THE C182 -- A DASH 8 AT 10 O'CLOCK POS AND 2 MI; CLBING THROUGH 8500 FT. THE C182 RPTED TFC IN SIGHT. SEPARATION HAD ALREADY BEEN LOST. THE DEP CTLR TURNED THE DASH 8 FROM 020 DEGS TO 120 DEGS. MY FAILURE TO ADVISE THE DEP CTLR OF THE UNTAGGED TFC LED TO THE ERROR. I HAD NOT NOTICED THAT THE TAG HAD DROPPED; BECAUSE I WAS COORDINATING WITH 2 OTHER SECTORS; ISSUED SEVERAL DSCNT CLRNCS; CLRED AN ACFT FOR APCH AT BFI; AND SHIPPED 1 ACFT TO FINAL. I WAS SIMPLY UNAWARE THAT THE SIT WAS DEVELOPING BECAUSE MY FOCUS FOR THAT MIN WAS ELSEWHERE. IN MY VIEW; TURNING A DEP N THROUGH ANOTHER CTLR'S AIRSPACE IN AN AREA (MAIN BANG OF ASR 9 ANTENNA) WHERE THERE IS A HIGH LIKELIHOOD OF UNTAGGED TFC BEING PRESENT; IS A DECIDEDLY HIGH RISK OP. I BELIEVE THAT PRE-COORDINATED CLB PROCS SHOULD PROHIBIT OPS WITHIN 5 MI OF THE ANTENNA DUE TO KNOWN EQUIP LIMITATIONS; COUPLED WITH CTLR WORKLOAD AT ARRS SECTORS DURING PEAK PERIODS. I BELIEVE I WAS AWARDED BLAME FOR AN ERROR THAT WAS PRECIPITATED BY POOR OPERATING PRACTICES BY ANOTHER CTLR AND SPECIOUS PROCS PUBLISHED IN A LCL FAA ORDER.