BJC CIC CTLR DESCRIBED POSSIBLE RWY INCURSION THAT WAS UNREPORTED BECAUSE OF FAILURE TO GAIN ALL NECESSARY INFO FROM OTHER CTLR'S.

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ACFT HAD LANDED ON RWY 20 AT BJC AND ADVISED HE PARKED AT THE TXWY L HANGARS. HE WAS INSTRUCTED TO TURN R AT TXWY A AND CONTACT GND. A FEW MOMENTS LATER; THE GND CTLR EXCITEDLY TOLD THE TWR CTLR 'STOP THAT GUY!' AND POINTED TO THE ACFT WHO WAS ABOUT TO ENTER RWY 29R AT TXWY A9. THE TWR CTLR TOLD THE GND CTLR THAT THE ACFT SHOULD BE ON GND FREQ. HE HAD PERFORMED SEVERAL CTL ACTIONS SINCE HE HAD LANDED; AND DID NOT IMMEDIATELY RECALL THE CALL SIGN OF THE ACFT; WHO HAD NEVER CALLED GND. I MOVED TO TRY TO LOOK AT THE TWR CTLR'S PAD TO HELP HIM RECALL THE CALL SIGN. WHEN I LOOKED AT THE ACFT AGAIN; IT HAD TURNED BACK ON COURSE ON TXWY A. SOON AFTER THAT; THE PLT CALLED GND; AND WAS ASKED TO CALL THE TWR ON THE PHONE. BECAUSE OF THE CONFIGN OF THE INTXN OF TXWY A AND A9; THERE IS AN AREA WHERE AN ACFT CAN TURN TOWARD RWY 29R AND BACK TO TXWY A WITHOUT XING THE HOLD LINES. I HAD THOUGHT I OBSERVED THAT THIS IS WHERE THE ACFT HAD BEEN; AND THAT THE ACFT HADN'T ACTUALLY VIOLATED THE RWY. SO WHEN THE PLT CALLED I DISCUSSED THE SIT AT TXWY A9 AND THE FACT THAT HE HAD STARTED TAXIING WITHOUT A CLRNC FROM GND; BUT DID NOT TREAT IT AS A RWY INCURSION. LATER; A CTLR WHO WAS DOING OTHER DUTIES IN THE TWR AT THE TIME OF THE INCIDENT INFORMED ME THAT WHILE I WAS LOOKING FOR THE CALL SIGN THE ACFT APPEARED TO HIM TO BE ACROSS THE HOLD LINE. AS CIC I SHOULD PROBABLY HAVE GATHERED MORE INFO FROM THE GND AND TWR CTLRS BEFORE TALKING WITH THE PLT ON THE PHONE; BUT THEY WERE BOTH STILL ON POS AND BUSY DURING THAT TIME.