Millions of viewers of the hit BBC TV drama series Sherlock failed to spot a vital clue that explains how the sleuth survived falling to apparent certain death, according to the show's creator, Steven Moffat.

Since Sunday's end-of-season cliffhanger there has been frenetic online speculation, but in an interview with the Guardian Moffat says no one yet has guessed what really happened.

"There is a clue everybody's missed. So many people theorising about Sherlock's death online – and they missed it."

At the end of Sunday night's second series finale, Benedict Cumberbatch's sleuth seemed to plunge to a sticky end after a struggle with his nemesis Moriarty. Later Martin Freeman's Watson delivered an affecting soliloquy at Holmes's presumed grave – unaware that Holmes (or someone just like him) was watching.

Moffat said he and co-creator Mark Gatiss wanted to baffle a nation by killing off Sherlock, putting his body apparently in plain view on a London pavement and then bringing him back from the dead.

Moffat claims he and Gatiss have gone one better than Arthur Conan Doyle in bringing Holmes back from the dead. "He cheated outrageously. He has Watson deduce that Holmes must have fallen off a waterfall. But there was no body. And it only means one thing in a detective show when there's no body."

The 50-year-old co-creator of the series is clearly revelling in wrongfooting the show's millions of fans. He previously tantalised viewers before the finale by suggesting that his hero might not survive.

Only after the episode was screened did he – via Twitter – and the BBC disclose that a third series had been commissioned.

But Moffat refused to explain precisely how Sherlock survived. That would be an elementary error for a writer who loves teasing audiences. "It's not a cheat. We've worked it out. It all makes sense." A third series will be screened, hopefully before the year's out, he said.

Until then fans will have to be content with devising theories – including one that Holmes turned into an enormous Sherlock-shaped robot spaceship controlled by tiny people (just as the Doctor cheated death in Doctor Who).