That is the question. Or at least it was for the managers of a mid-size Belgian company who, after their departure from said company, decided to work for the competition. They were sued on the basis of a legal principle called "good faith aftereffect" ("nawerking goede trouw") which is commonly relied on in Belgian corporate and labour law to prevent ex-employees or past directors from competing with their former employer or company for a
period of time, even in the absence of an explicit non-compete clause in their contract.
The reasoning is that because of the key strategic role they had, good faith alone would be sufficient to compel such managers from competing with their former company.
However, in a June 2020 ruling, the Belgian Supreme Court made short work of this notion, stating that in the absence of a clear legal obligation not to compete (either by law or by contract), economic freedom should prevail.
For former employees without a restrictive labour agreement, this judgement brings comfort in freedom. For many companies, it will be an incentive to check the (existence of) non-compete clauses in the agreements they have in place with their key personnel.