A private detective may, and usually does, become possessed of information even more valuable to the person who is being watched than to the person to whom he owes his allegiance.
Unreliable rascals constantly sell out to the other side and play both ends against the middle.

In this they resemble some of the famous diplomatic agents of history.

And police detectives employed to run down criminals and protect society have been known instead to act as stalls for bank burglars and (for a consideration) to assist them to dispose of their booty and protect them from arrest and capture.

It has repeatedly happened that reliable private detectives have discovered that the police employed upon the same case have in reality been tipping off the criminals as to what was being done and coaching them as to their conduct.

Of course the natural jealousy existing between official and unofficial agents of the law leads to many unfounded accusations of this character, but, on the other hand, the fact that much of the most effective police work is done by employing professional criminals to secure information and act as stool-pigeons often results in a definite understanding that the latter shall be themselves protected in the quiet enjoyment of their labors.