Petraeus and Broadwell apparently used a trick, known to terrorists and teen-agers alike, to conceal their email traffic, one of the law enforcement officials said.Rather than transmitting emails to the other's inbox, they composed at least some messages and instead of transmitting them, left them in a draft folder or in an electronic "dropbox," the official said. Then the other person could log onto the same account and read the draft emails there. This avoids creating an email trail that is easier to trace.
The trick has achieved notoriety as a tactic of terrorists who are rightly wary of espionage.
It's the sort of measure you take if you fear there's a risk that someone will look in on you. And it's been around for quite some time, which may be why the FBI investigators were not fooled by it.