When the security guard has the security chain fixed to his wrist he has a white bandage under his blue shirt sleeve to prevent chafing but in the police station, when the chain is unlocked to put on the policeman's wrist, also with a white bandage, the guard's wrist is bare.
An undercover cop calls Van der Valk on the Police HQ safe line, and says he has the name of the traitor. They arrange to meet somewhere for the cop to give Van der Valk the name--but the cop could've simply told him over the phone.