A census lists my ancestor as being born in "Canada ENG" - where is that?

That's difficult to answer as it has many meanings. The assumed answer would be Ontario, but that may not be entirely correct. It's safer to guess that it means your ancestor was born in Canada and their mother tongue was English. It doesn't narrow down the location but it also doesn't have you looking at just Ontario when you should also be looking at other English speaking areas of Canada (like the Maritimes). On the flip side if your ancestor was born "Canada French" assume it means they were born in Canada and their mother tongue was French (this again would not restrict your search to Quebec but to all French speaking areas)