Maybe the gap has an innocent (but undisclosed) explanation: someone "took time off, or went to school, or reengineered his life," Rosen says. But gaps can indicate bad things, too, such as a prison or jail stay.
Rosen says his favorite gap story came with an applicant who had an unidentified two-year absence on his application. When asked to explain it, he said he was in computer school. Where? Oh, in a state-sponsored program. Where was the program based?
Finally, he came clean. He'd studied computers at California's notorious San Quentin State Prison. Without inquiring about the gap, the employer might never have known about the incarceration.
Gaps are a problem because without knowing where someone lived, worked, or went to school, the employer won't be able to tell the background checking company which counties to search for criminal histories (criminal records are kept county-by-county).
"If we can verify where he was, we can be sure we're doing the search in the right counties," Rosen says. But a background check is nearly useless "if there's a period of time in which the person is unaccounted for," he says.