Keeping a 2nd home in Canada such as spouse or minor children while one spouse works abroad, I would think are required to file tax but parents both living abroad with adult children studying in Canada, I would think not but true, it is better to ask an accountant.
You're probably right, but CRA looks for excuses to declare someone a taxpayer. If the children were working adults (18 years of age or more), the husband would not be a taxpayer. However, what concerns me is that the younger one was attending school; I wonder whether there is some obscure rule that a child is deemed a dependent if he or she is attending school -- up until the age of 22.
In any event, the Statute of Limitations means that CRA could not go back more than 4 years and demand tax returns.