The 32-year-old Canadian, an electrical engineer, previously studied at McGill University in Montreal.
A friend in New Zealand expressed shock at the kidnapping but added that his humanitarian efforts seemed natural.
"That would not surprise me that he went to help somebody," said Allison Reay, manager of his school residence in Auckland. "He seems to get on well with everybody that he comes across ... I'm absolutely upset, floored."
Bill Williams, public relations officer at the Auckland school, declined to provide details on Sooden's activities there, saying "we're not able to give out information about students."