He may be arrogant, aloof and generally uninterested in the world beyond his career in crime solving, but Sherlock's focus on his work probably came at a cost. Refining his skills involved "sacrifices", reveals the show's star Benedict Cumberbatch – and we may be about to find out exactly what they were...

“He’s somebody who’s honed his craft and there are sacrifices involved in that," said Cumberbatch. "Some of those may be made clearer in the future of this series..."

Cumberbatch was speaking after a screening of season four opener The Six Thatchers, so fans may have to wait until later episodes to hear the particular revelations he's referring to. But what exactly could Sherlock have given up?

On one hand, it's hard to believe he's ever craved anything so mundane as a social life; on the other, we've watched Sherlock's friendship with John Watson, and his feelings for John's wife Mary and a growing circle of friends, burgeon as the series has gone one, so perhaps he did once hanker after a more 'normal' life than his work has allowed.

Could he even have given up a chance at love in order to pursue his calling? "Sentiment is a chemical defect found on the losing side," Sherlock once said, but it's not as if there have never been any women in his life. He was at pains to maintain that the frisson between himself and series two protagonist Irene Adler meant nothing, but actually it meant enough for him to travel to Karachi to save her from a group of terrorists. We've even seen Sherlock with a girlfriend of sorts – true, Mary's friend Janine was a means to an end but she also showed he is capable of doing 'relationship things' if necessary.

So perhaps there was once a woman in Sherlock's life (if so, please tell us she's going to turn up on his doorstep, or at least appear in some 'young Sherlock' flashbacks).

Or maybe this is a sacrifice relating to family – something to do with the third brother Mycroft Holmes once hinted at. "You know what happened to the other one," he reminded Sherlock at the end of series three. Maybe it's something Sherlock didn't want to be reminded of.

Whatever the case, this does prove one thing for certain – with so little information to go on, some peoples' imaginations are liable to run out of control...

Sherlock series four begins with The Six Thatchers on New Year's Day at 8:30pm on BBC1