If the alum doesn't appear in the repository, there is some chance the alum appears in bulletin boards; go there and search.
The most reliable source, however, is the grapevine. One thing you can try through this page is to post your query on the bulletin board. But the most reliable way is the old-fashioned way: look for friends and family of the person in question.
A few places you should not contact:
(At one time I kept a list of alumni that people had asked about. I removed it since I couldn't keep track of it very well, and since the bulletin boards are probably a better place to go anyway.)