ONE hot June morning in 1953, a retired couple from western Missouri packed their Chrysler New Yorker with 11 suitcases and started driving east. A few hours later, they stopped at a diner in Hannibal, Mo., and ordered fruit plates and iced tea.

“We thought we were getting by big as an unknown traveling couple until we went to the counter to pay the bill,” Harry Truman later wrote of that lunch. “Just as we arose from the table some county judges came in and the incog was off.”

What made Truman, less than six months removed from the presidency, believe he could travel incognito in the first place? It’s true that former presidents quickly drop from public consciousness. (Did you know that George W. Bush is preparing to throw the ceremonial first pitch at the Texas Rangers’ home opener? Or that Bill Clinton gave a speech to the European Union Parliament in Brussels last week — in the shadow of President Obama’s celebrated European tour?) But they remain famous, and surrounded by assistants and security agents.