FLT CREW OF MD80 DISCOVER THAT DEFERRAL OF THEIR INOP APU WAS NOT PROPERLY DOCUMENTED BY MAINT.

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UPON REVIEW OF THE LOGBOOK BOTH THE CAPT AND MYSELF NOTED THAT OUR APU HAD BEEN BOTH FOUND TO BE INOP AND DEFERRED BY MAINT IN STL. THIS; TO ME; SEEMED TO INDICATE THAT THE ACFT HAD STARTED ITS DAY AT THE HANGAR AND HAD FLOWN ONLY THIS ONE LEG INTO DFW. THERE HAD BEEN SEVERAL DISCREPANCIES WRITTEN AS CORRECTED IN STL BEFORE THE ONE PERTAINING TO THE APU; AND SO TO MY EYE EVERYTHING APPEARED TO BE IN ORDER. I IMMEDIATELY SET ABOUT MY NORMAL PREFLT DUTIES WITHOUT GIVING THE MATTER ANY MORE THOUGHT. BOARDING; ENG START; AND TAXI WERE ALL ON TIME AND NORMAL. IT THEN DAWNED ON ME THAT THERE WAS NO PLACARD PERTAINING TO THE NEED FOR A XTIE RELAY CHK TO BE PERFORMED PRIOR TO FIRST FLT OF THE DAY. WITH OTHER NORMAL CHKLIST AND TAXI DUTIES TAKING THE MAJORITY OF MY TIME AND ATTN; WE ARRIVED AT THE END OF RWY 35L AT DFW AND WERE #1 FOR TKOF BY THE TIME I HAD HAD ENOUGH TIME TO PROCESS AND WHAT I PERCEIVED TO BE STRICTLY PLACARD-RELATED PROB TO THE CAPT. I ALSO PULLED UP THE MEL AND HURRIEDLY SCANNED THE APU DEFERRAL VERBIAGE; BUT WAS UNABLE TO FIND ANYTHING RELATING TO THE NEED FOR A XTIE RELAY CHK PLACARD. ENRTE TO RSW; I BEGAN TO DIG DEEPER INTO THE MEL; DISCOVERING THAT THE VERBIAGE RELATED TO DEFERRAL OF THE APU AS A WHOLE UNIT ALSO STIPULATED THAT THE APU GENERATOR BE DEFERRED AS PRESCRIBED. THIS SECTION REFERS TO THE NEED FOR A XTIE RELAY CHK AND PLACARD INSTALLATION. THE CAPT AND I TOGETHER CAME TO THE CONCLUSION THAT THE ACFT HAD FLOWN FROM STL TO DFW WITHOUT A LOGBOOK ENTRY BEING MADE FOR THAT FLT AND THAT MAINT PERSONNEL IN STL HAD NOT INSTALLED A REQUIRED PLACARD FOR SAID DEFERRAL. IN RANGE OF RSW; WE ADVISED STATION MAINT OF THE PROB. TO MY KNOWLEDGE; THE RETURN FLT TO DFW LEFT ON TIME IN A FULLY LEGAL AIRPLANE; WITH THE POSSIBLE EXCEPTION OF THE MISSING LOGBOOK ENTRY FOR THE FLT FROM STL TO DFW.