The Beatles had a great deal of mythology and urban legend built around them. It was a parlor game for any fan of any band in the age before the internet. A rumor would start and develop, and be told from record store employees, radio disc jockeys, and a friend whose cousin's brother knew a guy, and it would spin out from there. The Beatles had a great deal of that when the band was together, and it really built up after they split up in a frantic quest for them to maybe get back together. Among those urban legends was a very, very persistent one that the Beatles had actually gotten back together in a secret reunion. Not only that, but they had released an album of new material without telling anyone that it was the Beatles.
Such a fate befell the band Klaatu, and their album
3:47 EST. They were not the only band, nor the only album to get this reaction, but they were the biggest result of that perpetual rumor. The album was not forthcoming with information, but it sounded like the Beatles so people believed it was the Beatles. Based on the rumors alone, record sales and word of mouth were enormous. And then the reality came crashing down.
http://ledgernote.com/blog/interesting/klaatu-the-secret-band-as-good-as-the-beatles/
Of course, there is also the fact that Paul McCartney, at the height of Pop fame, actually
did release an album which was secretly done by himself. The album
Thrillington, however, failed to do much of anything. So that may be a drag on the idea. Albeit, that album was in the easy listening and jazz genre, and held neither the style nor the interest of Beatles or solo career fans.
What if the rumor had eventually come true? What if at some point in the 1970s, the Beatles had reunited, recorded an album, and released it in secret without indication in the album credits or cover art or anywhere else that it was the Beatles? What if, though not the Klaatu album, a story like that had ended with the result of "Yep, it was us" and kept going?