Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (mon) 6
Communication Breakdown; Confusion; Distraction; Training / Qualification
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Flight Crew Executed Go Around / Missed Approach
Human Factors; Procedure
I was working the Ground Control position and CIC combined.  Aircraft X landed Runway 13R and was instructed to make a 180 and exit at A1; which is an intersection that does not cross Runway 13L.  I was making a new ATIS when Aircraft X called for taxi from A2.  It was sunset and when I scanned A2 I did not see the aircraft was between the runways.  I taxied him to parking and I realized he was crossing Runway 13L.  Another aircraft was in the pattern for Runway 13L and was on short final.  I did not stop Aircraft X as he was already clearing the runway.  Aircraft on final went around and Aircraft X was clear of the runway by the time aircraft on short final crossed the threshold. I asked LCW if Aircraft X was cleared to cross and LCW informed me Aircraft X was not instructed to cross nor did I give Aircraft X a clearance to cross.  I issued the Brasher Statement but Aircraft X did not respond.  I made several other attempts to contact Aircraft X on Ground Control frequency but never got a response. I should have confirmed he was at A2 and instructed him to hold short of the runway; although by the time he called for taxi he was already on the runway.
BFI Ground Controller reports of an aircraft taxiing across a runway that the aircraft was never cleared to cross.  The Ground Controller looked for the aircraft; didn't see it; and issued instructions.