Revealing mistake: To conceal their unauthorized entry into the secret lab, Cally very carefully closes and locks the sliding door. But when she turns back to speak with Tarrant, the "secured" door rolls halfway back open by itself. No, it's not Servalan's troops sneaking in to catch them: there's no sign of anyone there. Besides, anyone unlocking the supposedly-locked door would have alerted them. A case of malfunctioning BBC scenery, plain and simple.