What if No.6 is a plant, sent to the village to check on security, to check on No.2? That idea makes a total hash of how we see the opening sequence, that of a man who having resigned is trying to get away before they come for him. That would all be an act, as he allows himself to be abducted, as the Prisoner goes to work undercover in the village Absurd? Well Nadia Rakovski allowed herself to be abducted, to attempt a fake escape and be subjected to the awful effects of being captured by the village guardian! To make no mention of being interrogated and subjected to torture in that interrogation room, as she was a plant, working undercover in the village. Oh I know what Nadia Rakovski allowed herself to be put through is nothing compared to what No.6 was put through. But the principle is the same.
Certainly whoever No.,6 is he is definitely a spy. He has intelligence of working with codes, uses code names, Schmidt, Duval, D6, ZM73, XO4, and a false name of Peter Smith. He gives nothing away about himself, only that of his birthday 19th of March 1928, the only real thing we know about the Prisoner-No.6's identity.
It isn't until his supposed escape in The Chimes of Big Ben that we see that his cover is blown! the Colonel and Fotheringay have been brought to the village to help extract certain information from an old ex-colleague, and is left dumped in the village. Now he really is a Prisoner, betrayed by his very own people! But the Prisoner is still loyal, as we see in Many Happy Returns when the Prisoner "runs" again, back to old friends and ex-colleagues. And in Do Not Forsake Me Oh My Darling we not only learn that this one time spy, and undercover man is engaged to the bosses daughter, Janet Portland, and that the Prisoner's direct boss, above the Colonel is Sir Charles Portland. The fact that ZM73 had been sent on a mission deep under cover for the past year, means that even Sir Charles Portland was not privvy to such delicate information. After all, Sir Charles may have told his daughter under pressure by her!