WARNING: CONTAINS SPOILERS
Many people were surprised to see the return of Ryan Pilkington, who first appeared in the show as a young kid, back in series one. He was then one of Tommy Hunter's crew, delivering burner phones to shady types.
He later returned in the fifth season as a member of John Corbett's gang - up to no good once again, having slit the undercover cop's throat at the end of the series.
This time, he was at the Central Police force as a student police officer, kindly offering to make a cup of tea for DCI Davidson.
But Kate clocked that something wasn't quite right, asking if they'd worked together before - something he strenuously denied.
She's clearly onto the fella though, as she was seen looking him up on the police database.
Tweeting screenshots of the scene, one person wrote: "Kate knows!! #LineOfDuty did anyone else clock this??"
Kate knows!! #LineOfDuty did anyone else clock this?? pic.twitter.com/AqGXfbGEIG
- toni. (@ToniiWilliamsox) March 28, 2021
A second person commented: "I spotted this too!!! Let me at them!! #LineOfDuty."
Another fan replied: "What a spot. Thought you'd edited it, but been back and watched it again. Kudos."
While another comment went for a more direct approach: "HOW THE F*** DID I MISS THIS."
One other user wrote: "So when do you start work with the MI6 because this is next level. Damn."
#LineOfDuty Ep 2 thoughts.
1. Buckells interviewed Ryan in S1 so he should recognise him
2. Kate definitely recognises him cause she's looking him up!
3. Why is Johns wife back and why does she have loads of nice things.
4. DCI Joanne Davidson. Tommy's daughter?! pic.twitter.com/W2JuRagDoQ
- Natasha Haws (@HawbagHaws) March 28, 2021
The sixth season of the hit police drama made its triumphant return on 21 March, with seven episodes airing weekly on BBC One and on iPlayer.
Vicky McClure, Adrian Dunbar and Martin Compston have all returned as Detective Inspector Kate Fleming, Superintendent Ted Hastings and Detective Sergeant Steve Arnott, featuring alongside newcomer Shalom Brune-Franklin - who plays DC Chloe Bishop, a new addition to the AC-12 team.
Kelly Macdonald also debuts as guest lead Detective Chief Inspector Joanne Davidson, the senior investigating officer on an unsolved murder case whose suspicious conduct attracts the attention of Anti-Corruption.
Line of Duty series six continues on BBC One at 9pm on Sunday 4 April, and will also be available to watch via BBC iPlayer.
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