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As mentioned in the article, he was living in an apartment above an expat bar, on the very touristy riverside strip in Phnom Penh. It sounds like he wasn't really trying to hide. Perhaps because he was chronically ill, and needed to make frequent hospital visits anyway?

I think it's pretty unlikely, even if he did take various measures to hide, that he'd be able to if influential foreign governments were putting pressure on the Cambodian government to find him. Cambodia is a pretty tiny country.