Great question - the only safeguards that the government really has are:
-vetting employees via the "clearance" program, a giant anal probe of your entire life, family, friends, neighbors, employers, criminal record, travel, etc. (politicians, oddly enough, don't have to go through this process)
-the aforementioned random entry and exit personnel inspections (Think of a DUI checkpoint before you can leave work, but they're looking for classified documents instead of drunk drivers. Again, doesn't apply to politicians.)
-the honor system, depending upon employees to cop to their own mistakes ("If you 'accidently' remove a classified document you can bring it back and not get in as much trouble as if you would if we find out about it first." Again, doesn't apply to politicians.)
-the full-scope polygraph, where they grill you for an hour or two about any foreign contacts you might have had that you didn't self report, trips you took out of the country, whether you might have committed a security infraction and didn't report it (bringing a prohibited item into the SCIF or taking something out that shouldn't have been) and if you reported it or not (Again, doesn't apply to politicians.)
