20th and 21st century people are antsy-pantsy. We don't stay still. DC's musical patron saint, Duke Ellington, though born in the 19th century, was a 20th century man, moving around an about. Cultural DC Tourism's website mentions the various DC homes where Edward Kennedy "Duke" Ellington lived, including 1955 3rd St NW (pictured). However, there is no marker, no plaque. And I didn't discover this address through Cultural Tourism DC's site nor the Washington Post's map of Ellington places, but rather by his World War I draft registration card.
I figure as more archives, libraries and governments digitize and make their holdings available to the public the lives of 20th century people will be a bit clearer. Well, clearer than those folks wandering around in 1880-something. Not just of major notables, but of minor figures and regular folks. I found out E.L. (Euphemia Lofton) Haynes (she's got a charter school named after her) was a landlord for my house just by coming across the finding aid Catholic University Archives put up online. In the past Washington newpapers gave exact addresses for people, that's how I found the dead guy in my house.
So everyso often, check out a newspaper database, plug your addresss in Google, check out the 1940 census for yourself, or look at an old city directory. And as more an more material is digitized and indexed, the 20th century, the last 100 years will be easier to know and discover.