The most common threat that security and protection enterprises encounter is that of theft. It is in most cases impossible to recognize a thief before the plan to commit the crime is put into motion; even a suspicious and obviously out of place character could be just a decoy to draw the attention of the defenses in place away from the real theft as it is committed.
As is often the case, the worst damage is done from unexpected sources that are allowed unchecked past the defenses. The US Chamber of Commerce estimates that annual losses from employee theft are greater than equivalent period losses from burglary, robbery, and car theft put together. The same operational security procedures can help minimize both of these issues - constant vigilance in observation combined with complete accuracy and objectiveness in documentation will reveal many theft attempts before or as they happen, if the observer knows what to look for.