Ted got his written warning for unauthorised undercover investigations when he went a bit rogue last series. He posed as H to the OCG to try and uncover information after John Corbett disappeared (killed by the rogue PC Ryan after the OCG set him up and found out he was a rat).
Ted was trying to find information on the goods that had been stolen in the raid. Steve and Kate traced a phone to the warehouse where Ted was doing his undercover bit, and there was a big raid there.
Also finding Ted there on his unauthorised undercover operation.
He was arrested on suspicion of being H, and there were the issues with Gill Biggeloe trying to frame him. It came out he was in the clear of being H but was still disciplined for the unauthorised undercover operation.