What was found
MAIN CARGO DOOR LIGHT CAME ON DURING CLIMBOUT PASSING 15,000 FT MSL AT 310 KAUS, CHECKLIST ACCOMPLISHED, RETURNED TO AUS. AFTER AIRCRAFT BLOCK IN, OPENED MAIN CARGO DOOR. FOUND AFT THRESHOLD LOCK DOWN, WITH RED LIGHT ON. LOCKED THRESHOLD LOCK. CHECKED DOOR SWITCHES; CHECKED GOOD. CLOSED MAIN CARGO DOOR, WARNING LIGHT WENT OUT. PRECAUTIONARY PRESSURIZED AIRCRAFT TO 3.5 PSI ON GROUND. WARNING LIGHT DID NOT COME ON. NO LEAKS NOTED ON OUTSIDE OF MCD. MAIN CARGO DOOR WARNING SYSTEM OPS CHECKS NORMAL. AIRCRAFT PRESSURIZED PER 727 JRH CH 7. OKAY FOR SERVICE.