It is true that no countries have access to our criminal records system, which is why they have to ask the question in the first place. Now that the eTA application has been made, you can either contact the Canadian High Commission and tell them, or carry on as if you didn't have the caution. I'm guessing that you aren't a threat to national security and your caution wasn't collected to money laundering, organised crime, and so on, so they won't find out about unless you tell them. It would only become an issue if you think you will need a visa for Canada in the future, for work, study or residence, as your travel will be recorded against your name, so they will have your disclosure history available if you ever apply for a visa. If you never apply for a visa, it won't be an issue.