Shelf, mirror and picture hanging in Toronto usually means working with plaster and lath rather than tidy modern drywall. In a Cabbagetown row house or an Annex third floor, the surface is horsehair plaster over wood lath, and the studs behind it sit wherever the carpenter left them a century ago.
So I read the wall before I drill. A small pilot hole in a spot that will be hidden tells me more than any stud finder. Old plaster is brittle, but the lumber behind it is solid, and handled properly it will carry a heavy mirror for decades.