Originally Posted by MSPeconomist

AFAIK you are "required" to report a lost or stolen passport (although I'm not sure of the definitions and misplaced might not be the same as officially lost), but you can have more than one USA passport. (The concern is that someone else might fraudulently try to use or alter the stolen or lost passport.) People sometimes do this if they travel to countries where having stamps from other countries could be a problem or even if their travel patterns make getting visas difficult. I don't know what permission or procedures are needed for this.

Others maintain one official (i.e., diplomatic or for government business travel) passport and a second one for private tourist trips, perhaps keeping an old "private" passport for use after leaving government service..