Seo was last seen scuba-diving in the Sea of Cortez off the coast of Punta Prieta, Mexico, 700 miles south of the United States border. He was a spear fisherman and was hunting prey about 60 feet beneath the water's surface at the time of his disappearance. Seo never resurfaced. His diving companions found his speargun floating in the water. It had been fired. They followed the spear cord to a coral cave about 60 feet down, but there was no sign of Seo. He has never been heard from again.

Seo was employed as a mortgage banker in Norwalk, California at the time of his disappearance, and Los Angeles County, California police are investigating his case. He was married in 2004 and has two daughters. He is presumed to have drowned.