Having just started reading up on spies throughout history, I've just realized a new risk I hadn't thought of before, namely that once you've started spying you're at risk of being blackmailed into continuing to spy by your new masters even if you start to get cold feet and want to quit.
Has any spy managed to remain anonymous to their own handlers?
I'm aware that many spies want to be paid for their efforts, and that handlers want to pass on orders, both of which require 2-way communication, but even those challenges could be overcome with some imaginative thinking.
A spy who just wants to anonymously drop secret reports into the mail box should be a lot safer than one who has to surreptitiously loiter in foggy back alleys (or however they meet up)
I'm aware that many members of the 'Lucy' spy ring (WW2, Switzerland) are anonymous to this day, and I know that initial intelligence on the location and purpose of Peenemünde (a V2 rocket base) was passed to the British anonymously.
Are there other examples?
UPDATE - I'm aware that most agencies would like to verify the identity of their spies, but I'm also sure that if they receive the anonymous message "the enemy will attack at dawn" and the enemy does attack then the 2nd message will be taken more seriously.