I think a big part of the successful secrecy was only telling a VERY small group of people, and even though they were people who worked with him as David Bowie, they were truly friends with David Jones. Just like that Iman quote she fell in love with David Jones, David Bowie was just a persona. He might have been hard core into drugs but in the years since he married Iman and had his daughter he was a completely different, very private person. He'd walk his daughter to school every day. He took his wife and daughter to London to show them his childhood after he found out. David Jones was so unassuming that no one recognized him. I just think he kept his persona and his real life so far that he truly knew who he could trust until the end, and he was pretty much right.