The term private eye has widespread use to mean private detective or investigator. See, e.g., Oxford Dcitionary Online
Several websites, such as this one, suggest that the term was based on a logo adopted by the Pinkerton agency which featured an eye and the words We Never Sleep. However, none of those surces seem authoritative or provide any reference to a contemporaneous usage of the term.
Pinkerton registered the trademark We Never Sleep with an eye logo in 1950, claiming that it first used the trademark in 1884. [Search "we never sleep" at the USPTO trademark site.]
On its website, Pinkerton claims to be the worlds first private eye but does not claim to have originated, or explain the derivation of the term.
Numerous websites (again lacking authority) and at least one dictionary cite the derivation as
1935–40; eye, allusive phonetic rendering of [the letter] I, abbr. of investigator
Freedictionary.com [citing Random House Kernerman Webster's College Dictionary]
Etymonline lists the first use in 1938, but gives no explanation or source.
However, the term appears earlier in numerous books, including the 1930 Nancy Drew 04: The Mystery at Lilac Inn by Carolyn Keen
Try to figure this one out, Miss Private Eye! [Note: it is unclear as to whether this date is for the character or the actual text]
The term private eye is used to convey spying by Balzac, in the 1928 Cousin Bette
Lisbeth, whom the baron endeavored to ally with Madame Marneffe, so as to keep a private eye upon the household …
Similarly, it is used in The Judgment of the Sword: The Tale of the Kabul Tragedy, and of the Part Played Therein by Major Eldred Pottinger, the Hero of Herat in 1929
His fellow-commandant, Akbar's Master of Horse, had probably been sent to keep a private eye on the turncoat lest there were any attempt at rescue.
There are numerous earlier uses of the term for your private eye to mean for your confidential viewing.
From whence comes the term private eye to mean private decetive?