ISP GC ADVISES LCL THAT A C172 IS ON RWY 24 AFTER HEARING LCL CLR A BE55 FOR TKOF.

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I WAS WORKING LC AT THE TIME. IT WAS TWILIGHT. SUNSET WAS AT XA:25 LCL; INCIDENT HAPPENED AT XA:55LCL. RWY 24 WAS ACTIVE. I GAVE RWY 24 TO GC FOR A RWY CHK. ACFT ON 2 MI FINAL RWY 24. GC TAXIED BE55 OUT FOR A RWY 15R DEP. I PUT THE BE55 IN POSITION RWY 15R AFTER GC GAVE ME RWY 15R. GC HAD TAXIED A C172 TO RWY 24 INTXN DEP. THE C172 WAS DOING HIS RUNUP ON RWY 15R/33L WHEN I CLEARED THE BE55 FOR TKOF. I SCANNED THE RWY; BUT DID NOT SEE ACFT ON RWY. THERE IS A PILLAR 'POST' IN WINDOW AND ACFT WAS IN BLIND SPOT. BE55 CLRED C172 BY ABOUT 50 FT. GC ADVISED ME THAT THERE WAS A C172 ON RWY AFTER BE55 WAS AIRBORNE. BE55 ADVISED ME THERE WAS A C172 ON RWY ON DEP CLB OUT. LESSON: NEVER TRUST YOUR GC; AND GET OFF YOUR REAR END! THEN SCAN AROUND ALL OBSTACLES! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED HE ONLY RELEASED THE RWY FOR A VEHICLE RWY CHK TO DETERMINE SNOW OBSTRUCTION. GC DID NOT COORD WITH LCL FOR THE C172 USAGE. THE CESSNA 'HID' BEHIND THE POLE FROM WHERE THE RPTR WAS SITTING; ALLEGING THE CESSNA DID NOT HAVE HIS WING LIGHT ON; WHICH HE SAID WOULD HAVE REFLECTED OFF THE SNOW OFF THE RWY EDGE. RUNUPS ON RWYS 15 AT RWY 24 ARE VERY FREQUENTLY UTILIZED DUE TO THE LACK OF TXWY RUNUP AREAS ADJOINING THE RWYS.