Line of Duty fans were agog when one eagle-eyed viewer pointed out an interesting detail going back to the very first episode of series six.
Viewers took to Twitter to marvel after the fan saw what they believe could be a link to elusive fourth man known as 'H.'
It was a blink and you'd miss it moment in the BBC One drama about corruption in the police force, but it had fans wondering if it could turn out to be a major clue.
In the very first scenes featuring the mysterious DCI Joanne Davidson in her office was a sign that read "Please return all files once your finished with them Thank you!!!!"
Fans think this grammatical error might tie in with other errors made by the mysterious person who has been secretly messaging Davidson on her laptop during this series including the misspelling of definitely as 'definately.'
"Mate if you can spot that, you must know who H is. We want theories!" exclaimed one fan on Twitter.
"Wasn’t there a message sent to Jo Davidson on the laptop with grammar like this?" asked another.
"Probably had the sign at mates rates from the OCG printers," commented another.
In Line of Duty fans are aware there are no accidents or coincidences and the word 'definately' has cropped up during various character correspondences with a mystery OCG member believed to be H.
During episode five Jo got out her laptop to talk to her unknown criminal handler following Anti Corruption Unit 12's raid on a gun workshop.
The unknown handler wasn't happy that two OCG (organised crime group) henchmen were shot during the operation and told Jo that she'd have to get rid of DCI Kate Fleming played by Vicky McClure.
Jo, played by Kelly Macdonald, responded that she'd only do it if it was her final job, to which the unknown replied 'definately.'
Interestingly it was the same typo H used while exchanging messages with undercover officer John Corbett back in series five.
So Jo is likely to have been talking to the same person that John was speaking to last season.
