Actually we do know now. His light aircraft was transiting the English Channel, heading toward Paris and operating under visual flight rules (not under positive control). A formation of British bombers returning from an aborted mission -- recall that is was bad weather / fog that day, Dec 15, 1944 -- jettisoned their ordnance over the channel before heading to their home stations. An aircrewman in the formation recalls seeing one of the bombs hit a small aircraft below them -- almost 100% certain it was Miller's plane. Although the aircrewman reported what he saw to the flight debriefers, 2 and 2 were not put together until decades later.