MIA CTLR DISCOVERS THAT ACFT HE HAS CTL JURISDICTION OVER IS NOT RADAR IDENTED OR TRACKED.

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I WAS THE RADAR CTLR WORKING THE N DEPS OUT OF MIA AT THE 'D' SCOPE. ACR X; A BE1900 COMMUTER; WAS CLBING TO 15000 FT; CLRED OUT THE HEDLY TRANSITION (PHK 157 DEG RADIAL). I LOOKED FOR ACR X DATA BLOCK TO CHK HIS PROGRESS; BUT COULD NOT FIND IT. I SUBSEQUENTLY LOCATED AN UNTAGGED TARGET 15 MI WNW OF FLL BTWN 10000 FT AND 11000 FT. I CHKED THE ARTS AT MY SCOPE AND COULD FIND NO INFO ANYWHERE ON ACR X. THE UNTAGGED TARGET PROVED TO BE THE MISSING FLT. I CALLED THE SUPVR OVER AND TOLD HIM WHAT HAD HAPPENED AND THAT IT WAS VERY DANGEROUS; AND ASKED HIM TO FIND THE CAUSE SO WE COULD FIX IT. THE SUPVR TOLD ME THAT THE REASON IT HAPPENED WAS THIS: AT THE TIME ACR X DEPARTED MIA AND TAGGED UP; THERE WAS ANOTHER COMPANY ACR -- SAME CALL SIGN -- ON BASE TO LAND AT FLL. WHEN MY FLT TAGGED UP; THE FLL FLT'S TAG FROZE IN POS ON THE RADAR SCOPE. UNABLE TO MOVE; MODIFY; OR DROP THE FROZEN TAG THROUGH NORMAL MEANS; THE OTHER CTLR SUPPOSEDLY ENTERED 'TRACK DROP OK' ON HIS ARTS KEYBOARD; THEREBY APPARENTLY DROPPING BOTH TAGS. THE AREA WHERE I FOUND ACR X'S UNTRACKED TARGET IS ABOVE AIRSPACE BELONGING TO THE FLL N DEP CTLR ('L' SCOPE). LCL WRITTEN PROCS ALLOW THE 'L' CTLR TO CLB THROUGH MY AIRSPACE WITHOUT COORD; SUBJECT TO MY TAGGED TARGETS. THIS COULD HAVE BEEN DISASTROUS IF I WOULD NOT HAVE BEEN ALERT ENOUGH TO NOTICE THE 'MISSING FLT.' IT'S VERY POSSIBLE THE 'L' CTLR COULD HAVE CLBED THROUGH MY AIRSPACE WITHOUT REALIZING THE UNTRACKED COMMUTER FLT WAS THERE; AND 2 ACFT COULD HAVE GOTTEN TOGETHER.