Here's how my sister and I did it with our father who we presumed went missing Bogata, Colombia in 1952:
1. We had his service number from WWII and we had his SS #.
2. We asked other members of his family about him and if they heard word.
3. We looked in Science journals for pubs or letters. He was a scientist.
4. We checked former employers. He liked working foreign jobs.
5. We even checked U.S. death records from the LOC and Gov't.
6. My sister bought time on "Find Anyone". She gave them our information and they found him, or his death certificate, from Madrid, Spain. I deid of a heart attack on the street in 1968 while working as Systems Analyst for a computer company. He was 52. (I wasn't surprised as he was a former U.S. Marine with a hot temper, drank like a sailor and chain smoked these utterly deadly Turkish cigarettes- and was fat. He also hated sleep. So, guess what happened? Yup.)
The thing that made it a bit tough was that he teneded to get jobs where he wasn't supposed to go, like Oman, Cuba, the Arab Emirates and Taiwan. so, that's what we had to do. It only took us until 2009.