I don't know if anyone can say where CAPE FAIRWEATHER would have been located in Manila Bay/Cavite between December 4 and December 10, 1941. Depending on many factors (how busy the port was, what other ships had arrived earlier, what cargo she was carrying, what docks were available), the ship may have been docked anywhere in the harbor or even anchored in the harbor.
While in harbor, whether docked or at anchor, at least part of the ship's crew would remain on board to stand watch and carry out normal in-harbor duties. Other crewmen would likely be able to leave the ship on liberty for periods of time. Naval personnel on merchant ships in harbor during the war (meaning specifically Armed Guard crew) would have been aboard their ship to stand watch and would have manned the ship's defensive guns in the case of attacks or alerts. But again off-duty crew would likely have had some shore-side liberty. You do not state specifically that your father was in the Armed Guard, only that he was a navy radioman, but if he was in the Armed Guard he would have had to have been among the very earliest, since Armed Guard crews were placed aboard ships beginning only in November 1941.
One might have thought that in the first few days of World War II, confusion and concern would have led to ordering most crew to be back aboard their ships ready for any eventuality. But as I recall the Philippines were very unprepared for war in those early days. The Japanese air force attacked and destroyed many U.S. Army Air Force planes with impunity in the hours shortly after Pearl Harbor was attacked. And the Japanese army invaded the Philippines beginning on Dec. 22, 1941, encountering little effective resistance.
Curiously I can find no record of Armed Guard deaths or injuries anywhere or at any time in 1941 (see http://www.usmm.org/armedguard.html) nor can I find an indication that CAPE FAIRWEATHER was damaged while in the Philippines on December 10, 1941 (see http://www.usmm.org/sunk39-41.html#anchor447877). Of course it is possible that your uncle was killed in unusual circumstances, including perhaps being temporarily aboard a different ship or at a location ashore, that did not involve damage to CAPE FAIRWEATHER.
I am sorry not to be able to help you any more than the above.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website