As I said, CRA does have the authority to ask for any type of document. It could be the proof that they were tested for COVID-19. It could be an email from the employer saying that they came into contact with somebody at work and needed to isolate. It could be from the school saying that the school was closed so they have to stay home with their child. It could be any type of documentation.
On December 7th, 2021. See this statement in context.