Upon being in Indonesia, returning to HI and applying to college, he likely had lost it (especially with his transition to being a legal adult.). At some point thereafter, he gained his US passport by being a member of the Senate without having to jump through all the normal hoops. He has the passport of a citizen, but a paper trail that doesn't support same.
Being a Natural born citizen is yet another set of issues.
Obama travelled abroad in the 1980’s, long before he entered the Senate.
How did he get that passport?