PA44 PLT OBTAINS AN IFR CLRNC FOR DEP OFF AN UNCTLED ARPT AND DEPARTS WITHOUT AN ATC RELEASE.

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OBTAINED AN IFR CLRNC ON THE GND THROUGH THE MOORE COUNTY ARPT RCO (127.0); COPIED THE CLRNC AND READ BACK THE CLRNC TO THE CTLR. APPARENTLY; AT THE END OF THE CLRNC; THE CTLR HAD STATED: 'HOLD FOR RELEASE.' I EITHER DID NOT HEAR THE CTLR MAKE THAT STATEMENT OR MISSED WRITING IT DOWN. IN MY READBACK; I DID NOT REPEAT THE 'HOLD FOR RELEASE' INSTRUCTION; BUT THE CTLR REPLIED 'READBACK CORRECT.' I DEPARTED THE ARPT IN VMC AND IMMEDIATELY UPON CLBOUT SWITCHED FROM THE CTAF TO FAYETTEVILLE ON 127.3 MHZ AND STATED 'SEMINOLE CLBING THROUGH 1500 FT TO 3000 FT.' AT THAT MOMENT; THE CTLR REPLIED WITH THE QUESTION: 'WERE YOU GIVEN A RELEASE?' TO WHICH I REPLIED 'NEGATIVE...' AT THAT MOMENT; I REALIZED THAT I HAD MISSED SOMETHING. THE CTLR INQUIRED WHETHER OR NOT I HAD A FLT PLAN ON FILE. I REPLIED 'AFFIRMATIVE...AND I AM IN VMC.' THE CTLR RESPONDED 'STANDBY;' WITHIN ABOUT 5 SECONDS; REPLIED THAT I WAS 'RADAR CONTACT' AND CLRED ME TO 6000 FT AND DIRECT MY DEST. I REALIZE THAT I MISSED AN IMPORTANT INSTRUCTION AS A PART OF MY IFR CLRNC. I SHOULD HAVE OCCURRED TO ME THAT AT A NON TWRED FIELD; SUCH AS SOP WITH AN RCO AT THE FIELD; I SHOULD HAVE KEPT IN MIND THAT ACFT ARE TO HOLD FOR RELEASE; JUST THE SAME AS AT A TWRED FIELD (AS I DO EVERY TIME I FILE IFR). CONTRIBUTING FACTOR; BUT NO BLAME INTENDED AT ALL; WAS THE CTLR'S REPLY 'READBACK CORRECT;' WHEN MY READBACK WAS LACKING IN A PIECE OF CRITICAL INFO.