In these days when the US Intelligence services are receiving more scrutiny than they probably enjoy, I thought it might be interesting to look at how such things were handled in the Persian Empire.
The Persians had an intelligence service called The Eyes And Ears Of The King, which is a far more interesting and poetic name than the bland monikers you get these days. It sounds like some romantic, made-up thing, but I promise the Eyes and Ears of the King was a for-real organization, and not one you would want to mess with.
The Persian social structure was very hierarchical. At the top was the Great King. Directly below him were the Satraps, chosen almost always from Persian nobility. Each Satrap ruled a Satrapy, being a province, of which there were many. Each Satrap in turn had many officers in his province.
Everyone lived within the social heirarchy, obeying the next guy up the line, except for the Eyes and Ears. If you were a member of this elite organization, then your job was to keep an eye on how the Empire was ticking over, and report directly to the Great King, bypassing the entire system. Most important of all, the local Satrap had no power over you.
You kept an eye on how the local Satrap managed the army and put down rebellions.
You watched how tribute was collected from client states to make sure it all made its way to the King's coffers. (Satraps who enriched themselves were liable to rebel.)
If the taxation didn't add up, you investigated to find out who was diddling the accounts.
If a Satrap broke the law, you reported it to the Great King.
Any evil-doing going unchecked, you investigated, then let the Great King know.
The Eyes and Ears of the King was, in essence, the Persian FBI.
Xenophon tells us that in an emergency, an Eyes and Ears man had the power to command an army to move against a Satrap. I.e. to directly exercise the power of the Great King if he deemed it necessary for the safety of the state.
The Eyes and Ears were probably recruited from the most competent of the minor nobility, and surely were selected for their utmost loyalty. There are plenty of instances of Satraps moving against their King, but not a single record of an Eyes and Ears man turning rotten. To which it must be added, not a great deal was written about them in any case; they probably preferred to stay out of view.
I don't know of any Eyes and Ears man having an unfortunate accident while in a Satrapy, though you'd have to guess a lot of Satraps would have been quite happy to see the local agent drop dead. It's a fair bet that if it happened, the Great King would have an army on that Satrap's doorstep quick smart.
The Persians also used spies outside their empire. Herodotus says Darius sent a Phoenician spy ship to scout Greece before he invaded. On the ship were 15 Persian men of distinction. Some of those will have been young but highly competent military officers from noble families - their equivalent of today's special forces - and probably some of them were Eyes and Ears men, whose job was to notice things.