Also on a DC-10!
DFW to LGW, catering completed at aft r/hand door, safety strap left outside when the door was closed. Fast forward to initial cruise and a "thumping" noise coming from somewhere "at the back". I was the FO, formally an EO, and all three flight deck went back to listen and try to determine what the noise was. Not wanting to take this problem across the Atlantic, and JFK on a work to rule, we diverted to Atlanta. Fuel dump was delayed by a single dump valve failure. Landed, found the strap outside, refuelled and set course LGW. And yes ,we did tell the passengers the reason for the diversion.(lots of paperwork on arrival!)