ZSE CTLR FAILS TO MAINTAIN RADAR IDENT WITH PA28; ENTERS PDX APCH AIRSPACE AND CONFLICTS WITH ANOTHER PA28.

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ACFT X WAS CLRED FROM SLE TO HIO TO 4000 FT; RADAR IDENTED OFF THE RWY AND TURNED TO A 340 DEG HDG FOR TFC. ACFT X WAS SQUAWKING THE WRONG XPONDER CODE (ASSIGNED XXXX; SQUAWKED ZZZZ) AND NEVER AUTO-ACQUIRED. AS SOON AS I IDENTED THE ACFT (BASED ON THE TARGET RIGHT OFF THE RWY) AND TURNED HIM FOR TFC; I WENT ON TO OTHER DUTIES. THERE HAD BEEN A LOT OF ACTIVITY AT MMU ARPT JUST NW OF SLE; AND SEVERAL ACFT DEPARTED VFR TO PICK UP IFR CLRNCS. I GOT BUSY WITH TFC AND NEVER SAW ONLY THE LIMITED TARGET AND OBVIOUSLY DID NOT NOTICE THAT ACFT X HAD NOT TAGGED UP. THE ACFT CONTINUED INTO PORTLAND'S AIRSPACE WHERE ACFT Y WAS EXECUTING AN APCH INTO MMU. WHEN ACFT X WAS ABOUT 5 MI N OF UBG; PORTLAND APCH CALLED THE D-SIDE AND ASKED IF WE KNEW ANYTHING ABOUT A ZZZZ CODE. HE DID A FLT PLAN READOUT ON THAT CODE; AND SINCE IT WAS THE WRONG CODE; WE HAD NO INFO. BECAUSE WE HAD BEEN SO BUSY; THE D-SIDE DIDN'T CATCH THAT IT WAS OUR IFR AIRPLANE. ACFT X CALLED ME FOR A FREQ CHANGE AND THAT WAS WHEN WE REALIZED WHAT HAD HAPPENED. TO PREVENT SOMETHING LIKE THIS HAPPENING AGAIN; IT WOULD BE HELPFUL TO DEVELOP AN AUTOMATIC TRACK AS SOON AS AN ACFT IS DEPARTED. THEN THERE WOULD BE A VISUAL REMINDER THAT THE ACFT IS THERE AND SHOULD BE TAGGING. THE OTHER THING THAT WOULD HAVE BEEN HELPFUL WAS TO HAVE SOME HEADS UP THAT THERE WAS AN EVENT AND WHAT TIME THE ACFT PLANNED TO DEPART. WE DID GET THE PROPOSALS; BUT SOME PREPLANNED COORD WOULD HAVE BEEN HELPFUL.