When should I open the confidential envelope?

The confidential envelope should not be opened until you have reviewed all of the evidence in the case file and online. Your first tasks are to carefully work out the motives, means, opportunities, and alibis for each suspect / character. 

Usually, you should have two or three remaining suspects that you can't form concrete alibis for, this is where the confidential envelope comes into play. 

There may be twists, turns, and new evidence that emerges after opening the confidential envelope because the senior detective has been working on the case at the same time and has had access to other information.