In McCallum v Saputo ( 2021 MBCA 62 ) JA Pfuetzner made it clear that under the common law an employer has no duty to investigate prior to dismissing an employee although if they do not and just cause is not found they run the risk of additional damages for not doing so.
In other words, if you hired Sherlock Holmes as a workplace investigator who concluded that you did have just cause but the judge disagreed with Mr Holmes, then the employer would lose.
On the other hand if you did no investigation at all but proved your case in Court, then the employer would win.
Companies often spend huge amounts of money on workplace investigations but then are shocked to find out that they are of little or no assistance when it comes to actually proving their case in a court of law. The witness statements taken down by the investigator cannot be used as direct evidence in Court and even the Plaintiff’s statements can only be used to contradict what he or she says in Court. Of course the investigators assessment of credibility and or their conclusions on liability are not admissible as this is the exclusive function of the judge or adjudicator.