Employment Law questions: can post-termination conduct constitute just cause? 

To answer this question one must distinguish between after-acquired knowledge and post-termination conduct.  After acquired knowledge of misconduct that exists at the time of dismissal may be relied upon in support of summary dismissal.  Post-termination conduct can only be relied upon in very limited circumstances.

This principle was recently confirmed in Gillespie v. 1200333 Alberta Ltd, 2012 ABQB 105.

In this case the employee was terminated for personality conflicts at work.  It was later discovered that she breached the non-dislcosure agreement she signed.  The Court found that the post-termination breach of the agreement did not constitute grounds to support just cause.  Nevertheless, it also stated that the non-disclsoure agreement still had meaning.  It drew a distinction between the employment contract and non-disclosure agreement.  By doing so, the court was saying that the employer could not use the post-termination breach of the non-disclosure to justify just cause, but it be used to form a separate action on the breach of that agreement.