You have to admire the imagination that goes into fan theories, even if they are sometimes a bit far-fetched.

One that surfaced during a Q&A after the Sherlock finale screening on Thursday night was the possibility that there is actually a secret fourth episode in the current series, filmed at the same time as the first three and ready to be unleashed on unsuspecting (and some suspecting) viewers in the future.

The evidence for this theory? A clue apparently hidden in episode two, when villain Culverton Smith explains that he has confiscated three recording devices from Sherlock’s belongings.

“Must be something comforting about the number three,” says Sherlock. “People always give up after three...” before revealing that he has hidden another device in the walking stick John brought into the room earlier.

Knowing that they might not be able to get the increasingly busy Benedict Cumberbatch and Martin Freeman back together to shoot more Sherlock for the foreseeable future, did show creators Mark Gatiss and Steven Moffat take the opportunity to film an extra episode while they had the chance?

If so, when would it air? The schedules for Sunday 22nd January show that BBC1 has something called Apple Tree Yard on at 9pm. But then if they wanted to keep the secret, the Sherlock team would hardly advertise it in the listings would they? And anyway, perhaps they’re saving it as a surprise for later in the year.

So is this a possibility? Could there really be a fourth episode of Sherlock series four, a farewell gift to fans?

Well, no... not according to Steven Moffat, who was unequivocal when the question was put to him.

“It doesn’t reflect the fact that there’s gong to be a fourth episode – for heaven’s sake, what kind of secrets do you think we’re keeping?!

"Let’s go home and check. Oh look episode four… the lost special..."

But then, he would say that, wouldn't he...

Sherlock finale The Final Problem is on Sunday 15th January at 9pm on BBC1